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Water

2009-07-31T07:23:00.000-07:00

Think about the different ways you use water. You drink water when you are thirsty. You take a bath and wash your clothes with water. You water the grass or other plants. You swim in water. Water pouring over huge dams may even make the electricity that lights up your home. About three-quarters of Earth’s surface is water. Living things are mostly made up of water. Without water, there would be no life on Earth.

WHAT IS WATER?
Water is a chemical. Chemists say water is a compound, a combination of different materials. Water is a combination of oxygen and hydrogen atoms. An atom is a tiny bit of matter much too small to see. Water is made of one atom of oxygen and two atoms of hydrogen. The three atoms make the tiniest possible drop of water, called a molecule. Water can be a liquid, a solid, or a gas. Liquid water flows. Solid water is ice. Water in the form of a gas is called water vapor.

WHERE IS WATER FOUND ON EARTH?
Liquid water fills the ocean, lakes, ponds, rivers, and swamps. Water droplets form rain clouds. Liquid water makes beads of dew on the grass. It seeps down into the ground. It fills underground lakes and streams. Ice falls as hail or crystals of snow. Ice forms on ponds and frosty windowpanes. Huge sheets of ice make glaciers and icecaps at the North and South poles. Water vapor is always present in the air. Water vapor makes cloudsin the sky. Water vapor makes fog that hangs close to the ground. It is the steam that comes out of a teakettle.

WHERE DO WE GET DRINKING WATER?
Not all water is safe to drink. Water in lakes and rivers can be polluted. It can have harmful chemicals or germs that cause disease. Long ago, many people living in cities got sick or died fromdrinking dirty water. Scientists have learned how to clean, or purify, drinking water. They learned that boiling water could kill germs. They learned that adding certain chemicals could kill germs. Engineers learned how tokeep dirty water in sewers, away from drinking water. Governments passed laws to keep factories from polluting water. People still get sick from drinking polluted water. Many poor peoplestill do not have pure drinking water.

COULD WE RUN OUT OF WATER?
Nature recycles water all the time. Water in oceans, lakes, and rivers evaporates, or turns into a gas and rises into the air. The water vapor eventually turns back into a liquid and falls as rain. The water cycle keeps the total amount of water on Earth the same. But most of this water issalt water in the ocean. People need fresh water for drinking and for growing food on farms. Ocean water is too salty to drink. It is too salty to use for watering plants. Some places have more fresh water than others. People who live near big lakes or rivers have more fresh water than people who live in the desert. Places where a lot of rain falls have more fresh water. Places that usually have enough fresh water sometimes have a drought. Very little rain falls during a drought. People run short of water. People everywhere should be careful not to waste water.

Weights and Measures

2009-07-31T07:19:00.000-07:00

How tall are you? How much do you weigh? You can find out using weights and measures.

MEASURING LENGTH
Measurements of length tell how tall you are. The inch, foot, yard, and mile are units used for measuring length in the United States. This is called the English measurement system. There are 12 inches in a foot, and 3 feet in a yard. A mile is equal to 5,280 feet. Other countries and all scientists and engineers use the metric system. The centimeter, meter, and kilometer are metric units for measuring length. The metric system is based on units that can be multiplied or divided by 10. A centimeter is one-hundredth of a meter. There are 1,000 meters in a kilometer. You can measure how tall you are with inches and feet, or centimeters and meters. You also use feet or meters to tell how high a mountain or a building is. You measure longer distances with yards and miles, or meters and kilometers. There are special units for measuring the length of certain things. For example, the furlong measures lengths in horse races.

MEASURING WEIGHT
If you live in the United States, you can tell how much you weigh using a system of ounces and pounds. There are 16 ounces in a pound. The ton is the measurement for very heavy things, like
ships. A ton is 2,000 pounds! The metric system uses grams, kilograms, and metric tons. There are 1,000 grams in a kilogram, and 1,000 kilograms in a metric ton. You also use weight measurements to tell how much food you are buying. You use ounces and pounds, or grams and kilograms, to weigh potatoes, carrots, meat, and other kinds of food. There are special units for weighing other things. You use the carat to weigh diamonds and othergemstones. You use a different unit called the karat to measure the purity of gold.

MEASURING LIQUIDS
English units for measuring liquids are the teaspoon, tablespoon, fluid ounce, cup, pint, quart, and gallon. There are 3 teaspoons in a tablespoon, 2 tablespoons in a fluid ounce, 8 fluid ounces in a cup, 2 cups in a pint, 2 pints in a quart, and 4 quarts in a gallon. All those units can be a bit confusing! Metric liquid measurements are simpler. The most common metric units for liquids are milliliters and liters. There are 1,000 milliliters in a liter. You use the same units for all liquids from lemonade to gasoline. The barrel is a special unit for liquids. It is used to measure large amounts of liquid such as oil. A barrel of oil has 42 gallons.

President of the United States

2009-07-31T07:12:00.000-07:00

Have you ever dreamed of being president of the United States? You’ll have to wait until you’re 35, according to the U.S. Constitution. The Constitution also says you can’t be president unless you were born in the United States. The president is elected by voters across America. Before deciding to run for election, it’s a good idea to know what the duties and responsibilities of the president are. Although presidents have a lot of power, they also shoulder quite a heavy burden.

WHAT IS THE PRESIDENT’S JOB?
The president heads up the executive branch of the U.S. government. The executive branch is in charge of running the country and defending it from enemies. The president takes an oath to uphold the Constitution. The president is also responsible for carrying out the laws of the United States. Presidents suggest new laws. They appoint most of the country’s judges, including the judges of the Supreme Court. They are in charge of dealing with other nations. The president appoints ambassadors, who speak for America in other countries. The president also serves as commander in chief of the country’s military forces—the Army, Navy, Air Force, and Marines. Presidents cannot declare war—only Congress can do that.

Once a war has started, however, the president gives the orders. Every president has a group of advisers called the Cabinet. Each Cabinet member, except the attorney general, is called a secretary and heads a different department. The secretary of the treasury, for example, handles the country’s money supply. The attorney general is the chief law officer of the United States and heads the Department of Justice. Other secretaries deal with defense, schools, cities, and so on. There are 15 Cabinet posts in all.

HOW LONG DO PRESIDENTS SERVE?
A president is elected to serve for four years. The president may then run for one more term. When the nation was born, no law limited how many times a president could serve. By custom, however, no president ran more than twice. Then, in the 20th century, Franklin Delano Roosevelt ran and won four times. After Roosevelt died, a new law was added to the Constitution. It said presidents could serve no more than two terms.

CAN A PRESIDENT BE REMOVED FROM OFFICE?
Presidents who do serious wrong may be removed from office. First, the Congress must charge a president with wrongdoing. This is called impeachment. Then the president must stand trial in the Senate. If convicted, the president must step down. Two presidentshave been impeached, but none have been removed as president. Andrew Johnson was impeached in 1868 and Bill Clinton in 1999. One president, Richard Nixon, quit before he could be impeached. Five presidents have died in office. William Henry Harrison died of pneumonia one month into his first term. The other four—Abraham Lincoln, James Garfield, William McKinley, and John F. Kennedy—were killed by assassins.

HOW MUCH DOES A PRESIDENT EARN?
The president earns a salary of $390,000 a year. The president receives an additional $50,000 a year for expenses. The president’s salary was raised from $200,000 to $390,000 in the year 2000.

HOW DO PEOPLE GET TO BE PRESIDENT?
To become president it’s best to become the candidate of a big political party. A political party is a group that works together to achieve shared political goals. No law says presidents must belong
to a party. It’s just that it’s hard to win such a job without help. The United States has two major political parties, the Democrats and the Republicans. Every president since 1853 has belonged to
one of these two parties. Most people who run for president have first served as governor of a state or as a member of the United States Congress.

Some have come from the military. At one time, political parties chose their candidates at big meetings called conventions. Leading members of the party would get together and decide who should run. Now, most states hold primary elections before the presidential election. Citizens decide who should run from their party by voting in the primary. The political parties still have conventions. By then, their candidates have usually been chosen in state primaries. The conventions are mainly celebrations.

SOME GREAT PRESIDENTS
George Washington was the first president of the United States. He won election because he was a war hero. Washington led the American colonies to victory in the American Revolution (1775-1783). After his second term, many Americans wanted to make Washington king. He refused. In doing so, he kept American democracy going. Another great president was Abraham Lincoln. He took office in 1861. Lincoln led the country during the Civil War (1861-1865).

The war began after the Southern states broke away and tried to form a new country. Lincoln held the United States together and ended slavery. Many people consider Franklin Delano Roosevelt one of the greatest presidents. He was first elected in 1932. He took over during the Great Depression of the 1930s. This was a time when banks and businesses were failing and many people lost their jobs. Roosevelt used the government to create jobs and help the needy.During the 1940s, he led America to victory in World War II. Roosevelt greatly increased the powers of the presidency.

Telescopes

2009-07-31T07:00:00.000-07:00

Telescopes help us see things that are far away. They make distant objects look bigger. Using telescopes, astronomers have discovered thousands of stars, planets, moons, and many other extraordinary objects, such as black holes. The most common type of telescope is the optical telescope.This kind of telescope gathers light from distant objects.

HOW DO OPTICAL TELESCOPES WORK?
Imagine having eyes as big as your fist. You’d look funny, but morelight would enter your eyes. You would be able to see better.Telescopes bring extra light to our eyes. They effectively make oureyes bigger. Distant objects appear larger when you look through atelescope, and you can see more detail.A refracting telescope is the simplest type of optical telescope. It ismade up of two lenses. These lenses are similar to the lens in amagnifying glass. A reflecting telescope has a lens and a dishshaped mirror. The mirror collects and focuses (concentrates) light.A telescope’s eyepiece can be replaced by a camera. Then the image from the telescope is recorded on film or as a digital image.

GIGANTIC OPTICAL TELESCOPES
The bigger a telescope’s main lens or mirror, the more light the telescope gathers. The more light the telescope gathers, the more detail it shows, and the more distant the objects that you can seethrough it.Astronomers use huge telescopes housed inside buildings called observatories. These telescopes have mirrors as large as 26 feet (8meters) across. They gather enormous amounts of light.

BIG TELESCOPE WOES
Gigantic telescope mirrors are hard to build because they bendunder their own weight. When a mirror bends, it makes a blurryimage. One way to keep a giant mirror from bending is to divide
the mirror into smaller sections. Another way to avoid huge mirrorsis to use computers tocombine images from several telescopes. The Very Large Telescope in Chile, for example, has four telescopes with 26-foot (8-meter) mirrors. Together they gather the same amount of light as a telescope with a 52-foot (16-meter) mirror. Air causes another problem for telescopes. The air low in Earth’s atmosphere swirls about. This movement bends the light coming down from space just a bit, making the images we see through telescopes appear slightly blurry. To reduce this effect, large telescopes are often built on high mountains. This puts them above much of the air in the atmosphere. Many modern telescopes also have flexible mirrors. The shape of their mirrors can be automatically adjusted hundreds of times a second to adjust for the swirling atmosphere and keep the image sharp.

TELESCOPES THAT SEE INVISIBLE RADIATION
Optical telescopes are only one type of telescope. Astronomers also use telescopes that detect other kinds of electric and magnetic rays from space, such as X rays and radio waves. Our eyes cannot see these rays. Some objects in space aren’t bright enough to be seen with visible light. We wouldn’t know they exist without telescopes that can detect other types of radiation. A radio telescope, for example, detects radio waves given off by planets, stars, and other objects in space. It has a huge dish that collects the radio waves and focuses them on to an antenna in the
center of the dish. The dish can be turned to point at any part of the sky. The antenna turns the radio waves into electrical signals that astronomers record and study.

TELESCOPES IN SPACE
Several space telescopes are in orbit around Earth, beyond the atmosphere. From there, they have a perfectly clear view into space. This means they can see much more detail on distant objects. Some types of radiation, such as ultraviolet light, X rays, and gamma rays cannot pass through Earth’s atmosphere. Telescopes that detect these types of radiation must be launched into space.

THE FIRST TELESCOPES
We do not know exactly who invented the telescope, but we do know it was invented in Holland at the beginning of the 17th century. The first person to look into space through a telescope was the Italian scientist Galileo. He was the first to see moons orbiting Jupiter, Saturn’s rings, and mountains on the Moon.

Computers

2009-07-31T06:52:00.000-07:00

You’ve probably known about computers your whole life. But computers have not really been around for very long. Computers started to become popular with big companies in the 1960s. Computers didn’t become widespread in homes and schools until the 1980s.

HOW DO PEOPLE USE COMPUTERS?
People use computers in many ways. Stores use computers to keep track of products and check you out at the cash register. Banks use computers to send money all over the world. Computers help teachers keep track of lessons and grades. They help students do research and learn. Computers let you hook up to networks (many computers hooked together). They let you hook
up to a worldwide network called the Internet. Scientists use computers to solve research problems.
Engineers use computers to make cars, trucks, and airplanes. Architects use computers to design houses and other buildings. The police use computers to track down criminals. The military uses computers to make and read coded messages. Computers are not just desktops and laptops. Computers are everywhere around your home. There are tiny computers inside microwave ovens, television sets, and videocassette recorders
(VCRs) or digital video disc (DVD) players. There are even tiny computers in cars to help them run better.

HARDWARE AND SOFTWARE
Computers need hardware and software in order to work. Your desktop or laptop and all the parts inside are called hardware. The central processing unit (CPU) makes the computer work. The keyboard, mouse, printer, and monitor are also pieces of computer hardware. Memory chips are hardware that stores information and instructions. Information also gets stored on the hard disk drive. The programs that run the computer are called software. The ocmputer operating system is software that tells the computer how to run. Applications or programs are software that do certain tasks. Word-processing programs, for example, let you write school reports and letters.

HOW CAN COMPUTERS DO SO MUCH?
One reason that computers can do so much is that they have a special language that tells them what to do. Computer language has only two letters: zeros and ones. Computers can read these ones and zeros extremely quickly. Each zero or one is called a bit. Eight zeros and ones together are called a byte. Bits and bytes get stored in computer memory chips. Every year, computerengineers make chips that can hold more bytes. The chips can hold more information. Programmers can write applications that can do more things.

WHO INVENTED THE COMPUTER?
Many inventions have contributed to the development of modern computers. French mathematician Blaise Pascal and other inventors in the 1600s began making machines that could add and subtract numbers. Wheels, levers, and other moving parts made these machines work. In the 1800s, British mathematicians Charles Babbage and Augusta Ada Byron, countess of Lovelace, worked on plans for machines that could store information on cards with holes punched in them. American inventor Herman Hollerith made a machine that automatically totaled population figures for the 1890 United States census. His company joined with other companies to become International Business Machines (IBM) in 1924.

Other inventors built better computers. But none of these early computers were digital—that is, none used the digits zero and one. The first digital computer, called ENIAC, was built in the 1940s. It was huge. It was as big as a house. It had more than 18,000 glass tubes inside and weighed more than five elephants. The first computer used by business was called UNIVAC. Big
computers like ENIAC and UNIVAC were called mainframes. The desktop or laptop computer that you use today is much more powerful than those big machines. In the 1940s, scientists at Bell Telephone Laboratories invented a tiny electric switch called the transistor. In the 1960s, scientists and engineers invented integrated circuits or computer chips. Computer chips cram millions of transistors into a space the size of your little fingernail. Computer chips allowed computers to be smaller.

Personal computers (PCs) were invented in the 1970s. Most PCs are meant to be used by only one person at a time. They are small enough to fit on a desk. The Altair 8800 was the first PC. Apple Computer made its first PC in 1977. IBM made its first PC in 1981.

WHO INVENTED COMPUTER PROGRAMS?
Computer programs are sets of instructions that tell a computer what to do. Many people worked on early computer programs. The first programs were very hard to write and understand. They were extremely long strings of zeros and ones. American naval officer and mathematician Grace Murray Hopper in 1952 wrote the first program that turned English computer instructions into the strings of ones and zeros that make computers work. These programs are called compilers. In 1957, she helped develop the first programming language that companies could buy and use. It was called FLOW-MATIC. Hopper was also the first to use the word bug to mean a problem with a computer. She found a moth trapped in one of the computers she worked with. She taped the moth into her notebook and wrote, “First actual case of a bug being found.”

LATER DEVELOPMENTS
As computers have become more powerful and widespread, operating systems have become extremely complex. Few people can use a computer without one. Scientists at AT&T developed an operating system called UNIX in 1969. UNIX and related operating systems such as Linux are popular at universities and among computer professionals. In 1975, Bill Gates and his friend Paul Allen wrote a program for the Altair 8800 and founded the Microsoft Corporation. Microsoft later developed the DOS and Windows operating systems used on many home and office PCs. Computers keep getting smaller and more powerful. Personal computers that fit on a desktop today are more powerful than early “supercomputers” that filled entire rooms. Cell phones and watches contain tiny computers that can store information such as telephone numbers, addresses, and appointments. These devices allow you to surf the Web and play games. Many computer experts think that computers have only begun to make their mark on
history.

Paper

2009-07-31T06:45:00.000-07:00

What do a dollar bill, a cardboard box, and a book have in common? They’re all made from paper, of course! Paper is one of the world’s most important and useful products. Without it, there would be no newspapers, magazines, writing paper, or greeting cards. There would be no paper bags or boxes, paper money, gift-wrapping, or toilet paper. Take a look around you. How many things can you see that are made from paper?

WHAT IS PAPER MADE FROM?
Paper is made from tiny fibers from plants. You can see the fibers at the edge of a torn piece of paper. You can make paper from many types of plant fibers. Papermakers use fibers in straw,leaves, bamboo, sugar cane, and bark. Long ago, most papermakers used the fibers in cotton and linen rags. Today, most paper is made from wood fibers. The most important trees used for making paper are softwood trees. Softwoods include pine, fir, hemlock, and spruce. The long fibers in softwoods are ideal for making many kinds of paper. After paper is used, it can be reused, or recycled, to make new paper.

HOW IS PAPER MADE?
Paper is made in two stages. The first stage is to remove the fibers from the wood. This is done by grinding the wood or cutting it into chips that are softened with chemicals. The wood fibers are then mixed with water to make a souplike substance called pulp. The second stage is to spread out the pulp, press it flat, and dry it. This makes the fibers stick together in thin sheets. Some paper is still made by hand. But most paper is made by machines at factories called paper mills.

WHAT ARE THE DIFFERENT TYPES OF PAPER?
Different kinds of pulp make different kinds of paper. Pulp made by grinding is called groundwood pulp. It’s inexpensive to make, but the grinding breaks the wood fibers into very short pieces. Groundwood pulp is used to make cheap papers, such as newsprint. Pulp made using chemicals is called chemical pulp. The chemicals separate the fibers from each other but do not break them. Chemical pulp is used to make stronger, longer-lasting paper for use in fine books and magazines. The best writing paper and stationary comes from cotton and linen rag fibers. Thin rag fibers are long, strong, and make very durable paper. Fiber from recycled paper is used to make paper towels, napkins, and tissue. Paper for printing is treated with special chemicals so the paper won’t absorb ink and cause fuzzy lines that are hard to read.

HOW DOES A PAPERMAKING MACHINE WORK?
The main part of a papermaking machine is a wide belt made of tightly woven wire mesh. The belt moves in a loop, and it keeps moving all the time. Pulp is poured evenly onto the belt at one end of the machine. As the belt moves along, water drains from the pulp. The fibers remain, leaving a mat of wet paper. The belt goes through metal rollers that squeeze out more water. Now the paper is strong enough to be lifted off the belt. It passes between heated rollers that dry it completely. Finally, the paper is pressed tightly between cold metal rollers that make it smooth. The finished paper is wound onto large rolls or cut into standard sizes.

WHO INVENTED PAPER?
The ancient Chinese invented paper about 2,000 years ago. Chinese papermakers used fibers from tree bark and old rags. The art of paper-making spread out from China about 500 years later. It finally arrived in Europe about 900 years ago. The invention of the printing press in the 1400s made books popular, and the demand for paper increased. All paper was handmade until 1798. That’s when a Frenchman named Nicholas Robert invented a papermaking machine that could make paper in continuous rolls. Before the invention of paper, ancient people used many different surfaces for writing. They wrote on clay, wood, stone, and metals. More than 4,500 years ago, the ancient Egyptians made a paperlike material from a plant called papyrus. Papyrus reeds were cut into flat slices, layered, moistened with water, and pressed into sheets. The English word paper comes from the word papyrus.

Sound

2009-07-31T06:35:00.000-07:00

Your alarm clock goes off with a loud ring or buzz. You sing to yourself in the shower. You listen to your favorite music as you get dressed. You hear the wail of a siren on the way to school. You talk to some friends before class begins. You live in a world filled with sounds. All of these sounds seem to be so different. Yet all sounds share one thing—vibrations. All sounds come from something that vibrates. If you bang on a drum, the top of the drum vibrates. When you talk, the vocal chords in your throat vibrate.

VIBRATIONS AND WAVES
You can see how vibrations make sound by plucking a guitar string. The string vibrates back and forth. The vibrating string makes the air around it vibrate. The vibrations make sound waves in the air. The sound waves travel through the air to your ears. They make your eardrum and the inside of your ears vibrate. Your ears send a signal to your brain. Your brain tells you that you are hearing a guitar string.

You cannot see sound waves, but you can see water waves. If you drop a pebble in a still pond or a big tub of water, the pebble will make waves. You will see the waves go outward through the water in circles. Sound moves through the air in similar waves.

HIGH AND LOW
The sound of a whistle is different from the sound of a drum. The whistle makes a high sound. The drum makes a low sound. The highness or lowness of a sound is called its pitch. Whether a sound is high-pitched or low-pitched depends on how fast something vibrates. Fast vibrations make high-pitched sounds. Slow vibrations make low-pitched sounds.Fast vibrations make sound waves that are close together. Slow vibrations make sound waves that are farther apart. The spacing between sound waves is called the frequency. Scientists measure frequency with a unit called the hertz. One hertz is one vibration or sound wave per second. High-frequency sound waves make high-pitched sounds. Low-frequency sound waves make low-pitched sounds. Young people with normal hearing can hear sounds that are between 15 and
20,000 hertz.

LOUD AND SOFT
A police siren makes a loud sound. Whispering makes a soft sound. Whether a sound is loud or soft depends on the force or power of the sound wave. Powerful sound waves travel farther than weak sound waves. To talk to a friend across the street you have to shout and send out powerful sound waves. Your friend would never hear you if you whispered. A unit called the decibel measures the power of sound waves. The sound waves of a whisper are about 10 decibels. Loud music can have a level of 120 decibels or more. Sounds above 140 decibels can actually make your ears hurt.

MUSIC OR NOISE?
When you sing or talk, you send out sound waves with all kinds of frequencies. These sound waves mix together. How they mix makes music or noise. A chorus or choir sings in parts. The sounds of the parts mix well together. This kind of mixing is called harmony. Harmony makes the beautiful sounds of music. Honking horns and the motors of cars, buses, and trucks send out sound waves that do not mix well together. That is why traffic on a busy street makes awful-sounding noise.

THE SPEED OF SOUND
The speed of sound is how fast the sound wave travels. There is not just one speed of sound. At sea level sound travels through cool, dry air at about about 1,088 feet per second (about 332 meters per second). In warmer air, the speed of sound increases. In cooler air it slows down. Sound waves can travel through liquids, solids, and air and other gases. Sound moves faster through water and other liquids than it does through air. Sound moves fastest through solids. The speed of sound in steel is about 16,000 feet per second (about 4,880 meters per second).

ECHOES
Did you ever stand and shout in a cave, canyon, or big empty room? You would hear your voice twice. First you would hear yourself talk or shout. Then you would hear the same sound
coming from a distance. The sound waves of your voice go out until they hit a wall or other
surface. Then they bounce back toward you and make an echo. Bats use echoes when they fly around on dark nights. The echoes help keep the bats from flying into trees, houses, and other
objects.

HOW WE USE SOUND
People use sound for things other than talking and making music. Doctors use ultrasound to see inside the body. Ultrasound has frequencies too high for you to hear. Echoes from ultrasound waves can show what is inside the body. The Navy uses sound to “see” underwater. A detection device on ships called sonar sends out sound waves. The waves bounce back when they hit an object. Sonar helps sailors find submarines and other things underwater. Sonar also tells how fast and what direction things are moving. Scientists use sonar to make maps of the bottom of the sea.

Electricity

2009-07-31T06:26:00.000-07:00

Watch a bolt of lightning flash across the sky. Flip a switch and light up your bedroom. Click the remote and see the TV come on. What do all of these things have in common? Electricity.

WHAT IS ELECTRICITY?
Electricity is a powerful force of nature. Electricity is everywhere in the universe. Electrical forces hold water, metals, and all other kinds of matter together. You can walk and run because electric signals go through your nerves from your brain to your muscles. The signals tell your muscles where to move. Electricity makes many machines work. Electricity makes bulbs light up and runs motors in saws, fans, hairdryers, and other appliances. The computer you are using works because of electricity.

WHERE DOES ELECTRICITY COME FROM?
Electricity starts with atoms. Atoms are tiny bits of matter much too small for you to see. Everything in the universe is made up of atoms.Atoms have two main parts: a center or nucleus, and electrons that orbit or go around the nucleus. Electricity comes from electrons. You cannot see electrons and you cannot see electricity. You can see what electricity does because of electric charge and electric energy.

WHAT IS ELECTRIC CHARGE?
Electric charge comes from the parts inside atoms. There are two kinds of electric charge called positive charge and negative charge. Positive charge comes from the nucleus of an atom. Negative charge comes from electrons. Atoms do not normally have any overall charge because their positive and negative charges cancel each other out. Charge comes when electrons move away from an atom. Positive charge is just the opposite of negative charge. Positive and negative charges pull toward each other. The pull of positive and negative charges makes two kinds of electricity—static electricity and electric current.

WHAT IS STATIC ELECTRICITY?
Did you ever get a shock after walking across a carpet and touching a metal doorknob? That shock came from static electricity. Huge amounts of static electricity cause lightning. Electrons that move away from their atoms cause static electricity. You can make static electricity by rubbing certain materials together. Run a plastic comb through your hair. Be sure your hair is clean and dry. Electrons jump from your hair to the comb. This gives the comb a negative electric charge. Your hair loses electrons. This gives your hair a positive electric charge. Hold the
comb above your head and watch some of your hairs stand on end. Your hair stands on end because the positive and negative charges are pulling toward one another.

Static electricity also causes lightning. The pull of positive and negative charges between clouds and the ground creates a huge spark. The spark is actually the charges moving very quickly toward each other. Lightning can also be caused by opposite charges inside one cloud, between two clouds, or between clouds and the air.

WHAT MAKES LAMPS LIGHT UP?
Electric current makes lamps and all other electric devices work. Electric current is actually electrons moving in a big loop. Something must give the electrons a push to get them moving.Batteries can start electrons flowing. Batteries are a source of electric energy. A battery, two wires, and a light bulb can make an electric circuit. The current starts flowing from the battery through a wire to the light bulb. The other wire carries the electric current back to the battery. If you cut the wire, the electric current stops. Switches on an electric circuit turn the current on and off. This is how a wall switch works to turn lights on and off in your home. The electric energy in your home does not come from batteries. You plug appliances into electric outlets in your walls. The electric energy in the outlets comes from electric power plants.

HOW DO POWER PLANTS WORK?
Huge electric power plants generate or make electricity. Steam or falling water in dams make big machines called turbines turn. The turbine drives another machine called an electric generator. The generator makes electricity. Long power lines carry electricity from power plants to your home. Wires inside your home bring the electric energy to light bulbs, TVs, microwaves, and your computer.

WHO DISCOVERED ELECTRICITY?
For thousands of years people knew that a material called amber mysteriously pulled on some materials. The ancient Greeks called amber elektron. Scientists in Europe in the 1600s and early 1700s called the materials that amber attracted electrics. Benjamin Franklin, an American printer, patriot, and inventor, experimented with electricity. He thought lightning and electricity
were the same thing. He did a dangerous experiment in the mid-1700s to find out. Franklin flew a kite during a thunderstorm. He attached a metal key to the kite string. An electric charge ran down the wet kite string to the key. The charge made a spark when it hit the key.

This showed Franklin that lightning was electricity. He was lucky he was not killed. Many other scientists have experimented with electricity since Benjamin Franklin. They learned how to make electricity with batteries. They found that electricity would go through wires. An American inventor named Thomas Alva Edison invented many things that use electricity, including the electric light bulb.

Chemical Reactions

2009-07-31T06:23:00.000-07:00

Make both of your hands into fists. Put your closed fists together then pull them apart. Nothing holds your hands together, so it’s easy to pull them apart. Now link your fingers together. Curl them around one another and pull. It’s hard to pull your hands apart. Linkingyour fingers is like a chemical reaction.

MIXING CHEMICALS
A chemical reaction can happen when you mix two or more chemical elements together. A chemical element is made of only one kind of atom. There are more than 100 different kinds of atoms. Atoms are much too small to see. Atoms that make up chemical elements link together in chemical reactions. A chemical reaction makes a new kind of chemical substance. The new substance is different from the chemicals that made it. Chemists use chemical reactions to make all kinds of substances. They have made millions of new substances, including many kinds of plastics and medicines.

ATOMS AND MOLECULES
Atoms link up to make molecules. A molecule has two or more atoms. Chemical reactions make new molecules. Oxygen atoms take part in many chemical reactions. Oxygen and hydrogen atoms link up to make a molecule of water. The oxygen in a water molecule can link up with iron atoms in a car fender. The oxygen and iron react to form rust. Oxygen in the air reacting with carbon atoms in wood can make carbon dioxide. Carbon dioxide is a gas that plants need to live.

FAST AND SLOW REACTIONS
Some chemical reactions are very slow. Rust can take years to form. Other chemical reactions happen very fast. The flame you see when wood or paper burns is actually a fast chemical reaction. An explosion is an extremely fast chemical reaction.

Copper

2009-07-31T06:16:00.000-07:00

Have you used anything copper today? If you bought something and received change, there was copper in the coins. Did you use any electrical devices? The electricity was carried to your home by copper wires. There are even tiny amounts of copper inside you. Your body needs it for digesting food and keeping your blood healthy.

WHAT IS COPPER?
Copper is a reddish-yellow metal. When it’s found in pure form in the ground it’s called native copper. Usually, though, copper is found combined with other elements in rocks. These rocks are
called copper ores. When combined with other elements, copper is often greenish in color. The Statue of Liberty is made mostly of copper. Its greenish color comes from copper combined with the element oxygen from the air. Copper was one of the first metals discovered by human beings. People were making tools and jewelry from native copper over 10,000 years ago.

HOW COPPER IS USED
Pure copper is a soft metal. Early humans found that it made poor tools and weapons. They discovered that copper is much stronger when mixed with other metals. People made bronze by combining the metals copper and tin. They made brass by combining copper with zinc. Today, bronze and brass often contain other metals. But copper is still their main ingredient. Copper has long been used for making coins. Copper coins were always less valuable than silver or gold coins, because silver and gold are rarer metals. Most coins used in the United States today contain some copper. sheets were once used to cover the bottoms of wooden sailing ships. They kept the wood from rotting or being eaten by sea animals. Substances that contain copper are used to make blue-green inks and dyes. Other copper compounds are used as insect and weed poisons on farms or to purify water.

COPPER AND ELECTRICITY
Copper became more valuable in the late 1800s. That was when people discovered how to use electricity. Of all metals, copper is the second-best conductor of electricity. (Silver is better, but
copper is much cheaper.) Most copper mined today is used in the electrical industry. The wires in power lines are mostly copper. So is the wiring in electrical appliances and cords. Copper can be stretched into wires as thin as 0.001 inch (about 0.025 millimeters).

WHERE DOES COPPER COME FROM?
In ancient times, copper came mostly from the island of Cyprus in the Mediterranean Sea. (In fact, the name Cyprus means “copper.”) The ancient Egyptians, Greeks, and Romans made tools and weapons from Cyprus’s copper. Native Americans used copper too. They mined copper in what is now Michigan. Copper ornaments from this region were traded all over America. Today, much of the world’s copper comes from Chile. Arizona, Utah, and New Mexico are leading copper-mining states.

Air

2009-07-31T06:07:00.000-07:00

Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows. Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.

WHAT IS AIR?
Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon. Air also contains smaller amounts of hydrogen, carbon dioxide, water vapor, helium, and other gases. Oxygen is the most important gas for animals. Animals must breathe oxygen in order to live. Carbon dioxide is the most important gas for plants. Plants use carbon dioxide and sunlight to make food. Plants give off oxygen. Animals turn the oxygen back into carbon dioxide when they breathe.

TAKING AIR WITH YOU
You can go to places where there is no air. There is no air underwater, but you can dive underwater. You can stay underwater a short time just by holding your breath. Air tanks let you stay underwater for a long time. Scuba divers wear tanks on their backs. The tanks are filled with gases that make up air. The divers breathe the gases through hoses. There is less and less air the higher up you go. People gasp for breath at the tops of tall mountains. Airplanes must carry air. Once the airplane gets up high, air is pumped into the cabin where passengers sit. Astronauts have to take all the air they need with them—there’s no air in space!

Oxygen

2009-06-27T18:54:00.000-07:00

If an astronaut strikes a match in space, nothing happens. On Earth the tip bursts into flame because it reacts with a gas that is part of the air. That gas is oxygen.

WHAT IS OXYGEN?
Oxygen is invisible and has no smell, but it makes up about a fifth of the air around us. Oxygen is an element. Elements are basic substances that combine to form all the materials found on Earth. Water, for example, is a combination of the elements oxygen and hydrogen. Rust is oxygen combined with iron. Oxygen is the most common element on the planet. Almost half the
weight of Earth’s crust is oxygen, but the oxygen is combined with other elements in rocks.

LIFE NEEDS OXYGEN
Except for a few kinds of bacteria, all living things need oxygen. Without it they would die. Animals that live on land breathe oxygen from the air into their lungs. You are doing that right now. Plants that live on land take in oxygen through tiny openings in their leaves. Insects have tiny holes in their shells that allow oxygen to seep in. Animals and plants that live underwater absorb oxygen that is dissolved in water. Oxygen gets recycled from plants to animals and back again. Green plants combine the Sun’s energy with water and carbon dioxide to create food for themselves. In the process, the plants produce oxygen and release it into the air. Animals, including humans, breathe in the oxygen. Animals breathe out carbon dioxide. Plants use the carbon dioxide to make more oxygen.

HOW DO WE USE OXYGEN?
Anything that burns needs oxygen. When wood burns, it is actually combining with oxygen. The flame of a candle or a gas stove is produced by oxygen combining with other elements. Fossil fuels such as oil, coal, and natural gas also need oxygen so that they can burn. Burning, a process known as combustion, produces heat. We use the heat to keep buildings warm, cook food, produce electricity, and move our cars and trucks. Animals use oxygen to keep their bodies working and to move around. Their muscles need oxygen for energy. Hospitals give oxygen to patients who are short of breath. Some pilots and mountain climbers need to breathe oxygen from special tanks in order to travel at high altitudes where the air is too thin to breathe. Divers need tanks containing oxygen so that they can spend time exploring underwater.

Oil

2009-06-27T18:44:00.000-07:00

What would our world be like without oil? We wouldn’t have gasoline-powered cars. We wouldn’t have airplane fuel or oil to heat our homes. Many paints, fertilizers, and kinds of cloth are made partly from oil. So are many plastics, chemicals, building materials, and even medicines. It’s hard to imagine life without oil. Yet the world’s supply of this valuable resource is running out.

WHAT IS OIL?
Oil is the purified form of a black or brown liquid called crude oil. Crude oil is a mixture of substances called hydrocarbons. They’re called hydrocarbons because they are made up of the elements hydrogen and carbon. Hydrocarbons give off a lot of energy when they burn. That’s why they make good fuels. They also can be combined with other elements in many ways to make different products. Crude oil forms under Earth’s surface. It forms from the remains of sea animals and plants. (This is why oil is called a fossil fuel—it comes from fossils.) When these living things die, they sink to the sea bottom. Over millions of years, they form a thick layer. The layer can get buried under sand and rock. Pressure and high temperatures inside the planet change it into oil and natural gas.
Most crude oil is trapped far underground. But it seeps through to the surface here and there, forming pools of black liquid. People have known about these pools for thousands of years. In the 1850s, chemists began to discover different ways crude oil could be used. These discoveries set off a massive search for oil. This search is still going on today.

EXPLORING FOR OIL
Scientists and engineers explore for crude oil beneath the ground. They look for particular kinds of rock and land features on Earth’s surface. They explore underground with sound waves. But there is really only one way to prove that crude oil is present. You have to drill an oil well. Only about one-third of wells dug for exploration strike oil. The rest turn out to be “dry.” More oil sources have been found by lucky guesses than by science. Oil wells don’t keep pumping until the crude oil is all gone. Less and less oil comes up as a well begins to go dry. A well is only useduntil it costs more money to get the oil out than the oil can be soldfor. At that point the well is capped.

REFINING CRUDE OIL
To be useful, crude oil must be cleaned and purified. Then it must be separated into different substances. This process is called refining. Crude oil is refined at huge factories called oil refineries. Gasoline, fuel oil, asphalt, waxes, and other hydrocarbons can be separated from crude oil at various temperatures. Then, they can be processed for different uses.

A NONRENEWABLE RESOURCE
Oil is a nonrenewable resource. The supply is limited. It takes millions of years for oil to form. Once oil is used up, it’s gone. Nobody worried about this problem 100 years ago. But the world is
using more oil each year. Today, oil supplies about two-fifths of the energy used in the United States. Americans use about 700,000,000 gallons of oil every day (about 2,650,000,000 liters).
About two-thirds of it is used as fuel for cars, airplanes, trains, and other vehicles.

New sources of crude oil will probably be discovered. So will new ways of draining the last drops of oil from known sources. Even so, most experts agree that the world’s oil reserves will be largely gone by the year 2050.

Lighthouses

2009-06-27T18:40:00.000-07:00

Lighthouses are tall buildings near seacoasts. They shine flashing lights at night or in foggy weather. These lights guide ships that sail close to the coast. Many lighthouses also have foghorns to guide ships in foggy weather. Lighthouses are built at places on a coast that are important to ships. They warn ships of hard-to-see dangers such as rocks or strong currents. They also mark entrances to harbors or rivers. Most lighthouses are painted white so thatthey can be seen easily during the day.

WHEN WERE LIGHTHOUSES FIRST BUILT?
Lighthouses have been protecting sailors for thousands of years. Lighthouses were built on the coast of the Mediterranean Sea at least as long as 2,600 years ago. One of the so-called Seven Wonders of the World was an enormous lighthouse called the Pharos, built at Alexandria in Egypt. Ancient lighthouses were simple structures. Fires burned on top of them as signals to ships.

LIGHTHOUSE KEEPERS
Until fairly recently, many lighthouses had lighthouse keepers living in them. The job of the lighthouse keeper was to make sure the lights were kept in working order at all times. Sometimes the lighthouse keeper’s family lived in the lighthouse, too. It was lonely living in a lighthouse in an out-of-the-way place. The lighthouse keepers might not see any other people for weeks. These days, lighthouse keepers are no longer needed. The lighthouses work automatically.

LIGHTING A LIGHTHOUSE
Lighthouses of the past were lit by burning coal or wood. In the late 1700s and 1800s, oil lamps became popular. Many lighthouses burned whale oil, especially lighthouses in the United States. In the late 1800s, lighthouses began to burn gas in their lamps. Lighthouse keepers were needed to keep fires burning and clean up soot the fires created. Today, most lighthouses use high-power electric lights that rotate. Reflecting mirrors and lenses make the light beam stronger. The lights work much like the lamp on top of a police car, but they are much bigger and more powerful.

Glass

2009-06-27T18:35:00.002-07:00

Glass is a wonderful substance. It’s hard and firm, yet we can see through it. Glass can protect us from the wind and the rain. But it lets sunlight through, which is useful for keeping our homes light and bright. Glass is also brittle, which means it breaks easily. But there are ways to make it
strong.

HOW IS GLASS MADE?
Glass is made by melting silica, a chemical that comes from sand. At extremely high temperatures, silica melts and becomes liquid. Other substances are usually added to the melted silica to make the glass strong. In its liquid state, glass can be molded into different shapes. Glass was first made at least 4,000 years ago. About 2,000 years ago, people in the Middle East discovered that molten (melted) glass could be blown into different shapes. They put a gob of glass on the end of a metal tube. Then they blew air through the tube. The glass at the end of the tube expanded, just like a balloon. Glass blowing remained the usual method for making glass vessels until the early 1900s. In 1903, an automatic glass blowing machine was invented. Glass can also be made into sheets that are flat and smooth. It can be shaped in molds. Substances can be added to glass to give it color. And once glass is cold, it can be decorated by cutting and
painting. Glass can be recycled. It is sorted by color and then melted in big furnaces. The molten glass is formed into new shapes.

HOW IS GLASS USED?
Glass is quite cheap to produce because it is made from sand. Glass is used for making everyday items such as windows, bottles, and drinking glasses. Many beautiful objects are made of glass. Artists have produced elaborate shapes and elegant designs in glass. Stained-glass windows in churches are made of pieces of colored glass. The pieces are fitted together to form a picture. Glass can be made strong enough to stop a speeding bullet. Bulletproof glass is made by layering sheets of glass with sheets of plastic. Car windshields are made of shatterproof glass. They crack
rather than shatter if they are hit. This makes them much safer.

Colds and Flu

2009-06-27T18:29:00.000-07:00

You start sneezing. Your throat gets sore and scratchy. You have to blow your nose a lot. You don’t feel very well. Do you have cold? Or do you have the flu? Colds and flu are illnesses caused by germs. They are both caused by germs called viruses. But they are caused by different kinds of viruses. Colds are often called common colds. Flu is short for influenza.

IS IT A COLD OR THE FLU?
Signs that you have a cold or the flu are called symptoms. The symptoms of a cold and the symptoms of the flu are slightly different. The symptoms of a cold are sore throat, cough, sneezing, and a stuffy, runny nose. Colds usually do not cause a fever. Flu symptoms are like cold symptoms, but the flu also causes chills, fever, and headaches. It makes you feel tired and achy all over.
A cold or the flu usually lasts about a week. Every once in a while they can lead to a more serious sickness, such as an ear infection or a lung infection called pneumonia.

CATCHING A COLD OR THE FLU
People once thought you could catch a cold from getting a chill in cold weather. They thought that wet feet or drafts of cold air could give you a cold. We now know that germs cause colds and flu. The germs are passed from one person to another. They travel in coughs and sneezes. More colds and cases of flu happen in cold weather because people spend more time together indoors when it’s cold outside. It iseasier for germs to spread when people are close together.

IS THERE A CURE?
There is no cure for the common cold. People take medicine to help their sore throats, coughs, and runny noses. Doctors say that resting in bed is the best way to treat a cold. There is no cure for the flu, either. Doctors can give medicine to make you feel better. Resting in bed and drinking lots of juice and water is the best way to treat the flu.

AVOIDING A COLD OR THE FLU
You can get a flu shot to help keep you from catching the flu. A flu shot, or vaccination, helps your body fight off flu germs if they attack. But it doesn’t always work. The flu virus keeps changing. When it changes, the old vaccine no longer works. Doctors have to keep making new vaccines. For this reason, you need a new flu shot every year. Sometimes the flu changes enough in a single year that you can still catch it even if you’ve been vaccinated. There is no vaccine against the common cold because more than 100 different kinds of viruses cause colds. These viruses also keep changing.

One thing you can do to protect against cold and flu germs is wash your hands before you eat anything or touch your face. Your hands may pick up the germs from door knobs or other things touched by someone with a cold. Washing your hands thoroughly kills the germs. You can also try not to spread germs when you have a cold or the flu. Use tissues when you sneeze. Cover your mouth when you cough. And wash your hands frequently to keep from spreading cold germs to others.

Sleep and Dreaming

2009-06-27T18:22:00.000-07:00

Dogs sleep, cats sleep, and you sleep. All mammals and birds sleep. Scientists are not sure if fish, reptiles, and insects sleep. Big animals sleep less, and small animals sleep more. Elephants and giraffes sleep only 2 to 4 hours a day. Bats, opossums, and armadillos sleep 18 hours a day or more! Even kids need more sleep than grownups. A newborn baby sleeps 17 to 18 hours a day. A 10-year-old needs about 10 hours of sleep a night. Grownups need between six and nine hours of sleep a night. Some people need more sleep than others.

WHY DO WE HAVE TO SLEEP?
Scientists do not know for sure why you sleep. They do have some ideas. Safety could be one reason for sleep. People and other animals might sleep because it keeps them safe at night. It’s hard to see in the dark. Enemies could sneak up and attack animals that are wandering in the darkness. Most mammals and birds go to trees, underground dens, or nests at night. Prehistoric people went into caves or other shelters. They covered up with furs and fell asleep. You go into your home at night and snuggle up in bed. Sleep might also help your body work better. Things go wrong if you do not get enough sleep. It is hard to think and work and play unless you get plenty of sleep.

WHAT HAPPENS WHEN WE FALL ASLEEP?
Scientists have learned a lot about what happens when you lie down, close your eyes, and fall asleep. They study people in sleep labs. ometimes your eyeballs move back and forth while you’re asleep. They move fast. Scientists call this kind of sleep REM sleep. REM stands for rapid eye movement. Your body may twitch during REM sleep. Your brain is also very busy during REM sleep. It is almost as busy as when you are awake. What do you think your brain is doing? Here’s a clue: You dream during REM sleep. There is another kind of sleep. Your eyeballs do not move. Scientists call this NREM sleep. NREM stands for nonrapid eye movement. Your brain is not very busy during NREM sleep. You go back and forth between REM and NREM sleep all night long.

WHY DO WE DREAM?
Scientists have done many studies on dreams. They think your senses may have a lot to do with dreams. In dreams, you see and hear things. Dreaming is not like thinking about things. You have feelings during dreams. You may feel happy or angry. You feel fear if you have a nightmare. Your memories may have something to do with your dreams. Dreams are often like stories that stop before they are finished.

DO DREAMS MEAN ANYTHING?
People in ancient times looked for meaning in dreams. The ancient Egyptians believed dreams could tell the future. Some psychologists think that dreams show what people feel deep inside. They ask people to talk about their dreams. Some scientists think that dreams have no meaning. They think that dreams just come from nerve signals in your brain. Other scientists think that dreams are important for memory. They may help your brain sort out what to remember and what to forget.

Microscopes

2009-06-27T18:12:00.000-07:00

How is it possible for something right in front of your eyes to be completely invisible? It’s possible when that thing is too small to be seen with your eyes. We can see drops of water from a pond or lake. But we cannot see the thousands of tiny creatures that live in the water. Drops of blood contain tiny structures called cells, but we can’t see them either. Microscopes allow us to see invisibly small things.

Microscopes are one of the most important tools of scientists. Medical scientists use them to see the germs that make people sick. Biologists use them to see how plants and animals are constructed. Geologists—scientists who study the Earth—use them to find out what rocks are made of. Some microscopes are so powerful that they enable us to see individual atoms, the tiny building blocks of everything.

DIFFERENT KINDS OF MICROSCOPES
Some microscopes make tiny objects look larger using lenses that bend light rays. Microscopes that use light to look at small objects are called optical microscopes. The simplest optical microscope is a magnifying glass. The best magnifying glasses can make things look 10 to 20 times larger than they actually are. The optical microscopes used by scientists are called compound microscopes. They contain several lenses. The scientist looks through a lens called the eyepiece to see the magnified object. Compound microscopes often have three different settings. Each setting provides a different magnification. Optical microscopes can produce magnifications of up to about 1,000 times. To view even tinier things, scientists use microscopes called electron microscopes and scanning probe microscopes. An electron microscope shoots tiny particles called electrons at the thing being viewed. Instruments turn the pattern of scattered electrons into an image on a screen. The best electron microscopes can produce images with magnifications up to 1 million times. A scanning probe microscope uses an extremely tiny probe, or tip, to “feel” the surfaces of tiny objects. The end of the probe might have a width of just one atom. A scanning probe microscope can make images of the individual atoms on the surface of an object. Some scanning probe microscopes can magnify objects by as much as 100 million times.

WHEN WERE MICROSCOPES INVENTED?
Historians think the compound optical microscope was invented by a Dutch eyeglass maker, Zacharias Janssen, in the late 1500s. Most microscopes were not very good until the early 1800s. That is when lens makers first learned how to make lenses that produce really clear, sharp images. German scientists developed the first electron microscope in the early 1930s. The first scanning probe microscope was created by Swiss and German scientists in 1981.

Lasers

2009-06-05T07:09:00.000-07:00

Lasers are powerful enough to cut through steel. Lasers are delicate enough to use in eye surgery. Lasers “read” the information coded on compact discs (CDs). These are just a few things that lasers do.

WHAT IS A LASER?
A laser is a device that produces a beam of light and makes the beam more intense. A laser beam is very exact. It can travel a long distance without spreading out and losing its power. Laser light is unlike sunlight or light from a light bulb. To understand the difference, think of a crowded city street. Thousands of people are walking along the sidewalk. Their clothes are of many different colors. They are walking in many different directions. Now think of a marching band in a parade. Everyone is wearing a uniform of the same color. They are all walking in the same direction, in step with one another. Sunlight and light from lamps are like the crowd on the sidewalk.

They are made up of many colors—all the colors of the rainbow. They spread out in all directions from their source. Laser light is like the marching band. It is light of a single color. It travels in a
beam. It spreads out very little, even when traveling through outer space.

HOW ARE LASERS USED?
Because lasers produce such exact beams of light, they are very useful tools. Some lasers are so powerful they can drill holes in diamonds. These lasers can cut a piece of steel to an exact shape needed for a machine part. Powerful lasers can produce temperatures of 10,000° Fahrenheit (5500° Celsius) and higher. This ability makes them useful in factories for joining together large
pieces of metal. Much smaller lasers “read” price tags on products. At the supermarket, you’ve probably seen the checkout clerk run foods
over the laser scanner. Small lasers in compact disc (CD) players read the information coded on CDs. This information is then played back as music. Lasers even carry telephone conversations. Laser beams send thousands of telephone calls through thin glass threads called optical fibers. In hospitals, doctors use lasers for delicate operations such as repairing damaged eyes. The military uses lasers to guide airplanes and missiles. And laser beams are used to create colorful light shows. These are just a few of the ways we use lasers.

WHEN WERE LASERS INVENTED?
Famous scientist Albert Einstein first suggested the idea of a laser in 1917. In the 1950s, scientists began making the idea work. American scientist Gordon Gould suggested the name laser in 1957. It was short for Light Amplification by Stimulated Emission of Radiation. The first working laser was built in 1960. It was built by another American scientist, Theodore Maiman. The development of lasers advanced rapidly during the 1970s and 1980s. Scientists today are using a huge and powerful laser to study how atoms join together in the Sun to release energy. This laser was built at Lawrence Livermore National Laboratory in California. It takes up a building the size of a football stadium.

Hair

2009-06-05T07:03:00.000-07:00

Hair is one of the things unique to the mammal world. Dogs, cats, lions, squirrels, and seals—they all have hair. So do humans. Hair grows only on mammals.

WHAT IS HAIR?
Hair is an extension of your skin. It’s made up of old skin cells that are filled with a protein called keratin. Pack a lot of these cells in a long, narrow line and you have a hair. Keratin is used to make fingernails, claws, scales, beaks, feathers, and other skin attachments, too. Each hair grows inside its own follicle. A follicle is a little pit in your skin. And hair grows at the root, not at the tip. That means that new hair is added to the bottom, or root, of the hair and the old hair gets pushed up. That’s why when dyed hair grows out, the dyed part gets pushed farther from the scalp and the real hair color appears at the bottom. In humans, hair comes naturally in black, brown, blonde, red, white, and various shades in-between. In animals, there are even more varieties and patterns of hair color. Markings such as a tiger’s stripes and a leopard’s spots come from differences in hair color. They are often how we quickly identify these animals.

HAIR HAS MUSCLES?
Yes! Each hair follicle has a tiny muscle called the arrector pili attached to it. When you’re cold or when you’re scared, the muscle pulls the hair up, making your hair stand on end. That’s what
goose bumps are: the muscles in your hair follicle pulling tight! Have you ever seen a cat hiss and arch its back? It’s the same thing. In animals, the raised hair makes the animal look bigger and
scarier than normal so that a would-be attacker might run away in fright.

THE JOB OF HAIR
Hair does a lot of other things, too, depending on the animal. When hair is real thick it’s called fur. Fur keeps an animal warm. Some animal fur is even valued by people because it is so warm.
Fur color can help an animal blend into its surroundings, allowing it to hide better. Some animals even change color with the seasons. The arctic fox, for example, is dark in summer but white in winter. This winter coat makes it hard to see the fox in the snow. Even the spines of a porcupine are modified hairs. Spines are just stiff, sharp hairs that protect the animal when attacked.
Hair also helps animals sense their surroundings. Longer hairs can brush against things to help animals orient their bodies. For example, whiskers are special hairs around the face that help an
animal feel its way around in the dark.

HAIRSTYLE
In humans, hair’s importance is mainly as decoration. Hair and beards have been a great part of dress and style for both men and women since ancient times. The way kings and queens wore their hair influenced the fashion of the day, the same way that a celebrity hairdo today can start off a hairstyle trend.

Dams

2009-06-05T06:56:00.000-07:00

Beavers build them from sticks. Landslides create them from trees, mud, and debris. Humans make them from earth and concrete. These structures are dams. Dams hamper the flow of water in a river or stream. Landslides don’t mean to create dams. They do so by dumping a lot of earth and other stuff in a river. Scientists think beavers build dams for protection. Beaver dams capture water in front of the lodges in which beavers live. Beavers can hide from their enemies in this deeper water. A dam also protects the beaver lodge by slowing the river’s speed.

WHY DO WE BUILD DAMS?
We build dams to control water. A dam built across a river or stream stops the water’s flow. Water collects in a lake behind the dam. The lake stores water for people to use later. The lake, or water storage area, is called a reservoir. The water in reservoirs travels in pipes to people’s homes for drinking water. It can flow through canals for farmers to use in watering their crops. People also sail boats and swim in reservoirs. Many dams use reservoir water to produce electricity. Water flows into large machines called turbines inside the dams. The turbines power other machines that generate electricity. Electricity produced in this way is called hydroelectric power.Some dams are built to prevent flooding. During the rainy season, the reservoir stores the river’s extra water. During the dry season, the dam sends the reservoir water back into the river.

HOW BIG ARE DAMS?
If you’ve ever visited a large dam, you know it is an amazing sight. Dams are some of the biggest structures ever built. The Hoover Dam on the border of Nevada and Arizona is as tall as a 72-story building. The Grand Coulee Dam in the state of Washington contains enough concrete to build a sidewalk all the way around the Earth. Few dams are this big, however. Most dams are small structures less than 10 feet (3 meters) tall.

WHAT ARE DAMS MADE OF?
Many large dams are made of concrete. Some are made of packed earth or rocks. Because these materials are not as strong as concrete, dams made of earth or rocks must be very thick. The Tarbela Dam in Pakistan is made of earth and rock. It contains more than 15 times as much material as the Grand Coulee Dam.Dams must be strong enough to withstand the pressure of water against them. Dams also must be cared for and repaired. A dam that breaks can cause disaster. In 1889, a dam in Pennsylvania broke and let loose a wall of water. The water submerged the town of Johnstown, knocking down houses and killing more than 2,000 people.

Bows and Arrows

2009-06-05T06:52:00.001-07:00

Thousands of years ago, human hunters had a problem. They couldn’t get close enough to an animal to use spears or knives to bring down their prey. Finally, they found a clever solution: the bow and arrow. No one knows just how or when the bow and arrow was invented. Some experts say it was developed more than 25,000 years ago. No matter when it appeared, it changed hunting and warfare for good. Using a bow and arrow, humans could strike at animals and enemies from a long distance away and with more accuracy. People didn’t have to get so close that they might be killed first.

THE BOW AND ARROW IN WARFARE
The bow and arrow was a valuable weapon for many armies. The ancient Egyptians were the first people known to have used the bow widely in war. The Romans, Chinese, Persians, and other civilizations also used the bow and arrow against their enemies. During the 400s AD, the Huns from Asia rode into Europe on horseback. The Huns terrorized Roman armies with powerful composite bows. Composite bows consisted of several materials glued together, which made the bows more elastic. These bows were shorter than standard bows and handier to use when fighting on horseback. They could kill with great accuracy from up to 300 yards (275 meters) away. That distance is as long as three football fields! In the 1200s, another Asian people, the Mongols, also made great use of the composite bow. The longbow is one of the most famous bows in history. English bowmen used it with great skill. An arrow from a longbow could
pierce a knight’s armor or kill his horse. The legendary English hero Robin Hood was said to be so skilled a bowman that he could split one arrow with another arrow. The longbow required strength, but it could fire an arrow hundreds of yards with great accuracy and force. England fought a lengthy war with France in the 1300s and 1400s. During battle, thousands of arrows shot from English longbows rained down on French knights in armor, killing many.

By the 1500s, armies had started to use guns in warfare. As guns got better, they completely replaced the bow and arrow. But bows were still used in some parts of the world. Native Americans fought against white settlers with bows and arrows. The Great Plains Indians had powerful bows that could fire an arrow all the way through a buffalo at close range.

THE BOW AND ARROW IN SPORT
Today, people no longer use the bow and arrow in warfare. But they do use it in hunting and in the sport of archery. Many states and Canadian provinces have hunting seasons for archers to hunt animals with a bow. Archers also take part in sports competitions. Archery is an event in the Olympic Games. Some modern bows are still made of wood. Others are made of carbon or fiberglass, which makes them lighter and more flexible. The compound bow has a system of cables and pulleys attached to it. The cables and pulleys make it easier to pull back the bowstring, allowing the arrow to go farther. Most arrows today are made of aluminum, fiberglass, or carbon fibers. The fletching, or feathers, on the end of the arrow may be made of turkey feathers or plastic. Fletching makes an arrow flystraight after leaving the bow.

Robots

2009-06-05T06:46:00.000-07:00

When you think about robots you may imagine metal machines from science fiction that look a lot like people. There are already almost a million robots at work in the world. Almost none of them look like the robots in science-fiction movies. Robots are machines. They are machines that are controlled by computers. Robots do work. You probably would not like to do the work that robots do. Some robots do jobs that are dangerous. Some robots do jobs that are boring. They just do the exact same thing over and over again. Robots that do these jobs are called industrial robots. Almost all robots in use today are industrial robots.

WHAT DO INDUSTRIAL ROBOTS LOOK LIKE?
Most industrial robots are just mechanical arms. Robot arms can bend. Some robot arms bend like an elephant’s trunk. Some robot arms can make themselves longer or shorter. Many robot arms have parts on the end that can hold things. The parts are called grippers. They work like a human hand, but they often don’t look much like a hand. Special kinds of grippers can handle tools or move things around.

HOW DOES A ROBOT ARM MOVE?
Your muscles move your arm. Electric motors move a robot’s arm. A robot arm has joints that allow it to bend just as your arm does. Your arm has shoulder, elbow, and wrist joints. A robot arm can have as many joints as it needs to do its job. A computer figures out how the robot’s arm and gripper should move. The computer sends signals to the electric motors. Some robot arms have sensors. The sensors tell the computer where the arm is. The computer makes the motors move the arm if it is not in the right place.

WHAT KINDS OF JOBS DO ROBOTS DO?
Robots do things over and over in exactly the same way. The robots can move quicker than humans can, and they never get bored. Most robots are designed to do only one specific job. A different robot must be specially made for each job that needs to be done. Many robots work in plants that make automobiles. Robot arms weld metal car parts together. They spray paint on cars. Other robots work in factories that build radios, TVs, computers, and other electronic products. Some robots help doctors do operations. Robots help replace hips. They help doctors operate on eyes. Some robots handle chemicals that are dangerous for humans to touch. Some robots go to dangerous places. Robots can go deep underwater to search for sunken ships or look for minerals to mine. Robots can go into active volcanoes. Robots help explore Mars and other planets. They find out what the planets look like and what they are made of. Robot rovers that look like little wagons landed on the planet Mars. They rolled around and examined the rocks and soil.

WILL ROBOTS EVER BE LIKE HUMANS?
Scientists and engineers are working to make better robots. They are trying to make robots with computers that are smarter. They are trying to make robot legs that walk. It is very hard to make a machine that can walk on two legs the way you can. One day there will be robots that make highways and build steel skyscrapers. Inventors are starting to make robots for use at hometo clean carpets and mow lawns. There may someday be robots that help with many chores around the house. Tiny robots may one day be able to go into clogged blood vessels and clean them out. Tiny robots may be able to go inside broken machines and fix them. Very smart robots may eventually be able to run a whole factory by themselves.

Plastic

2009-06-05T06:40:00.000-07:00

What is as hard as stone, strong as steel, clear as glass, light as wood, and springy as rubber? Plastics can be like all of these things. Plastics are materials used to make a wide variety of products. Plastics cost less than many other materials. Plastics do not rot like wood. They do not rust like metals. Plastics can be any color. You use plastics everyday. Your computer case is made of hard, strong plastic. You drink out of cups made of light plastic foam. You wear clothes and walk on carpets made of plastic threads. You buy food and other goods covered in clear plastic wrap. Plastic is everywhere around you.

WHERE DOES PLASTIC COME FROM?
Plastic comes mainly from fossil fuels. Oil, coal, and natural gas are fossil fuels. The building blocks of fossil fuels are the chemicals hydrogen and carbon. Like all chemicals, hydrogen and carbon are made from atoms, tiny bits of matter much too small to see. Everything is made of atoms. You make plastic by heating fossil fuels and adding other chemicals. The hydrogen and carbon atoms link together. The atoms join together like beads on a string. The long chains of atoms are what make plastics special. Metal, glass, wood, and clay are made of short chains of atoms.

MOLDING PLASTIC
Plastic starts out as a hot, sticky goo, called resin. The resin gets made into car parts, compact disks, plastic bottles, and all the different kinds of plastic products. To make plastic products, the heated resin is poured or forced into a mold. The resin cools and hardens into the shape of the mold. In fact, the word plastic comes from Latin and Greek words that mean to mold.

DOES PLASTIC HARM THE ENVIRONMENT?
People in the United States throw millions of tons of plastic away every year. Plastics do not easily break down, or degrade. Plastics fill up garbage dumps, called landfills. Plastics are a major source of pollution. Using less plastic helps reduce the amount of plastic waste. Making thinner plastic bottles and other containers uses less plastic. Many plastics can be recycled, or reused. Recycled plastics are used to make new plastic products. Recycling reduces the amount of plastic that gets dumped in landfills.

Nuclear Weapons

2009-06-05T06:20:00.000-07:00

First you see a blinding flash of light, brighter than the Sun. Moments later, a huge ball of fire appears, brilliant orange. The fireball begins to rise into the sky. Soon it widens at the top and is shaped like a mushroom. A thundering sound and blast of heat reach you 15 miles (24 kilometers) away. You are seeing the explosion of the world’s first nuclear weapon, on July 16, 1945, in a New Mexico desert.

WHY DO WE HAVE NUCLEAR WEAPONS?
Nuclear weapons are the most destructive weapons ever made. Building a nuclear weapon was a top-secret project during World War II. Scientists had been working on this weapon—the atomic
bomb—for three years by 1945. Almost nobody else, except the president of the United States, knew about this work. The secret effort to build a nuclear weapon was called the Manhattan Project. By 1942, when the Manhattan Project began, Germany had conquered much of Europe and was out to conquer the rest. The United States had just joined the war. The United States and its allies were afraid that Germany would develop an atomic bomb first. Then Germany would win the war. The United States and its allies had to beat Germany to the bomb. Germany had already surrendered by the time the atomic bomb was ready. But Japan was still fighting the war. To end the war quickly, the United States dropped two atomic bombs on Japan. The bombs killed at least 100,000 people and destroyed the cities of Hiroshima and Nagasaki. Japan surrendered soon afterward. The nuclear age had begun.

WHY WAS THERE A NUCLEAR ARMS RACE?
The nuclear arms race was a buildup of nuclear weapons after World War II. When the war ended, scientists knew that it was possible to build nuclear bombs far more powerful and destructive than the first atomic bomb. Some people, including scientists, thought it was wrong to build these weapons of mass destruction. Others feared that the Soviet Union would make them first. By the late 1940s, the Cold War pitted the United States and its allies against the Soviet Union and its allies. Each side feared an attack from the other side, though their armies did not actually fight during the Cold War. Everyone knew that a war using nuclear weapons would be a terrible disaster. A nuclear war would kill millions of people and possibly end life on Earth. Each side believed that having a large supply of nuclear weapons would frighten the other side and stop it from starting a nuclear war. If one side attacked, the other side would strike back with even more nuclear bombs. And so began a race to have more nuclear weapons than the other side. Luckily, no nuclear attacks happened after World War II.

HOW DO NUCLEAR WEAPONS WORK?
A nuclear weapon gets its name and its explosive power from the nucleus (core) of an atom. Atoms are tiny building blocks of matter much too small to see. An atomic bomb works by fissioning (splitting) the nuclei of atoms of the metals uranium or plutonium. It is sometimes called a fission weapon. A hydrogen bomb works by fusing (joining together) the nuclei of atoms of the gas hydrogen. Atomic bombs and hydrogen bombs are the two main kinds of nuclear weapons. The hydrogen bomb is far more powerful and destructive than the atomic bomb. The hydrogen bomb is like a tiny star. It works by the same process—the fusion of hydrogen atoms—that makes the Sun and other stars shine. A nuclear weapon destroys by the power and heat of its blast. The atomic bomb dropped on Japan flattened buildings within 3 miles (5 kilometers) of the blast. Heat from the bomb caused fires and burned everything near the place it exploded. People’s skin was burned as far as 11 miles (18 kilometers) from the blast site. A nuclear weapon also releases harmful radiation. People near thelast can die of radiation sickness even if the bomb doesn’t kill them. People farther from the blast may develop cancer and other illnesses from radiation months and years after the bomb explodes.

THE FUTURE OF NUCLEAR WEAPONS
No one has used a nuclear weapon in war since the United States dropped atomic bombs on Japan in 1945. For some years, countries tested their bombs underground or in remote places. However, test-ban treaties have halted the testing of nuclear weapons. The Cold War ended in the 1990s. It left the United States and Soviet Union with huge numbers of nuclear weapons.Other countries also have built nuclear weapons. The large number of nuclear weapons has produced new fears. What if a terrorist or an unstable government gets hold of a nuclear weapon? This possibility continues to frighten people.

Christopher Columbus

2009-06-05T06:05:00.000-07:00

Christopher Columbus tried to take a shortcut, and ended up somewhere he never intended to go. He discovered two continents that people in Europe didn’t even know existed. By crossing the Atlantic Ocean in 1492, Columbus opened contacts between lands and peoples that were unknown to each other.
Columbus’s voyage to the Americas opened an exciting period in history. Animals, plants, and new ideas were exchanged between continents. But it also caused terrible tragedy. Millions of Native Americans died as Europeans rushed to take land and riches for themselves.

MASTER SAILOR
Christopher Columbus was born in 1451 in Genoa, Italy. He became a sailor at the age of 14. In 1476, he was shipwrecked off the coast of Portugal. Portugal was Europe’s top seafaring nation at
that time. Columbus settled there. Columbus studied geography and navigation, the science of
figuring out where things are on Earth’s surface. He became a master sailor. He met explorers who had sailed along the coast of Africa seeking an eastward sea route to the rich lands of Asia.
Europeans called these lands “the Indies.” Europeans wanted to bring gold and other treasures from the Indies back to Europe.

DARING DREAM
Columbus began to think about a wonderful adventure, which he called the “Enterprise of the Indies.” He dreamed of reaching the Indies by sailing west! This was not a new idea, but no one had ever managed to make the voyage. Columbus thought the trip to the Indies west across the ocean would be much shorter than sailing around Africa.

Columbus had high hopes, but no money. Who would pay for his expedition? He asked the king of Portugal, but the king refused. Columbus didn’t give up. He went to the rulers of Portugal’s neighbor, Spain. At first they also refused. Eventually, however, the Spanish king and queen agreed to provide three small ships—the Pinta, the Niña, and the Santa María. They also paid for crews and supplies for the voyage.

HISTORIC VOYAGE
Columbus sailed from Palos, Spain, on August 3, 1492. He stopped at the Canary Islands southwest of Spain, then headed west into unknown seas. He had no idea what lay ahead, but he had faith in his sailing skills and his bold idea. A swift current carried his ships along, and on October 12, the crew sighted the islands of the Bahamas. Columbus thought he had reached Asia. He called the islands the Indies.

Columbus was greeted by the Arawak people who lived on the islands. They offered food, but had only a little gold. Columbus was disappointed not to find Asian treasures, but still felt sure he had reached Japan in Asia. He spent two months exploring, then headed home. One of his ships sank in a storm, but back in Spain he was hailed as a hero. The king and queen offered rich rewards and made him “Admiral of the Ocean Seas.”

THREE FAILURES
Columbus made three more voyages to America. None went well. He was a skillful sailor, but his greed and stubbornness made him a bad leader and created enemies. During his second voyage (1493-1496), Columbus claimed land for Spanish settlements. He fought against Caribbean peoples who lived on the land he claimed and forced them to work as slaves. On the third voyage (1498-1500), Columbus quarreled with Spanish settlers so violently that he was sent back to Europe as a prisoner in chains. On his fourth and final voyage (1502-1504), Columbus was marooned on an island for more than a year. He had to be rescued. He was very ill by the time he returned home to Spain.

AN EXTRAORDINARY EXPLORER
Columbus died in 1506. He quarreled with the king and queen right up until his death. He wanted authority over Spanish colonies and a larger share of the riches that were brought back from America. It was a sad end to an extraordinary career that still shapes our lives today. When Columbus crossed the Atlantic, he changed the world forever.

Olympic Games

2009-05-15T16:20:00.000-07:00

Every four years, athletes from around the world come together to compete in the Olympic Games. Do you know why this event is called the Olympics? The games were first held at Olympia in ancient Greece. The ancient Olympic Games honored the Greek god Zeus. Today, the Olympic Games are held in different cities around the world.

THE ANCIENT GAMES
We know that the ancient Olympics began as far back as 776 BC. That’s when the Greeks began keeping records of the winners. Theancient Games continued until about AD 392, more than 1,000 years! Athletes came from cities throughout Greece to compete in races, boxing and wrestling matches, gymnastics, and weightlifting. They also threw spears, hurled a discus (bronze disk), and jumped for distance. Wealthy Greeks raced their horses. Winners were crowned with wreaths of olive or palm leaves.

The ancient Olympic Games were not just a sporting event, however. There were competitions in poetry, music, speechmaking, and other arts as well. At the beginning and end of the Games, animals were sacrificed (killed and offered) to Zeus. A splendid temple was built at Olympia in Zeus’s honor. When people stopped worshiping the Greek gods, the Olympic Games were canceled.

THE MODERN GAMES
The Olympic Games were brought back in 1896. The first modern Olympic Games were held in Athens, the capital of Greece. Only nine countries participated in the first Games, and all the athletes were male. Today, some 10,000 athletes compete in the Olympic Games, and nearly half of them are female. They represent about 200 nations. At first, the modern Olympics included only summer sports, such as swimming, rowing, and track and field. Figure skating was
added in 1908, and ice hockey in 1920.

The first winter Olympic Games were held in 1924. More winter sports were later added to the Winter Games, including downhill skiing, bobsledding, and ski jumping. Snowboarding and freestyle skiing followed in the 1990s. From 1924 through 1992, the Winter Games and the Summer Games took place in the same year. After 1992, the next Winter Games were moved up two years, to 1994. Winter Games and Summer Games now occur two years apart. Each of these Games takes place every four years.

GOING FOR THE GOLD
After each Olympic event, medals are awarded to the competitors who finish in first, second, and third place. First-place winners receive a gold medal. Those who finish in second place receive a silver medal, and those in third place, a bronze medal. Olympic athletes often dazzle the world. In 1912, Jim Thorpe of the United States won the gold medal for two of the most difficult contests in track and field: the pentathlon, which consists of five different events, and the decathlon, which consists of ten events. Thorpe is still the only athlete to have won the pentathlon and decathlon at the same Olympics. In 1932, Babe Didrikson of the United States became the only Olympic athlete ever to win medals in separate running, jumping, and throwing events. Four years later, African American track star Jesse Owens won four gold medals. During the 1970s, the thrilling performances of Olga Korbut of the Soviet Union and Nadia Comaneci of Romania inspired a generation of girls to take up gymnastics. Also in the 1970s, American Mark Spitz amazed the world by winning a total of seven gold medals in swimming. Sarah Hughes in 2002 charmed audiences as she skated her way to a gold medal.

Nelson Mandela

2009-05-15T16:10:00.000-07:00

Nelson Mandela, a boy from an African village, grew up to become the first black president of South Africa. Before he became president, Mandela led a long and difficult struggle against segregation in South Africa. Under segregation, black and white people were kept apart. Segregation denied blacks many basic rights. Mandela spent many years in prison for trying to end segregation in South Africa.

EARLY LIFE
Nelson Rolihlahla Mandela was born in 1918 in a small village in the Transkei region of South Africa. His father was a chief of the Thembu tribe. Mandela’s parents named him Rolihlahla, an African word that means troublemaker. Little did they know how fitting his name would be! At the age of seven, Mandela became the first person in his family to go to school. At school, Mandela was given the name Nelson. He went on to attend collegeand earn a law degree in the city of Johannesburg.

FIGHTING SEGREGATION
When Mandela was a young man, South Africa was divided by segregation. Segregation in South Africa was called apartheid, a word that means apartness. Under apartheid, black people couldn’t vote or hold certain jobs. Whites controlled the government. Blacks and whites lived in separate areas and went to different schools. Mandela opposed this cruel and unfair system. In 1944, Mandela joined a group called the African National Congress (ANC). The ANC opposed the rule of South Africa by whites alone. The ANC believed that South Africa belonged to everyone, whatever the color of their skin.

A NATURAL LEADER
Mandela was a natural leader and a gifted speaker. He became a leader in the ANC, and he encouraged people to break the apartheid laws. The government saw Mandela as a troublemaker. It tried to stop him. The government made the ANC illegal. Mandela was arrested several times. When he was released, he continued to fight for an end to apartheid. In 1962, the government sentenced Mandela to five years in prison. Then, in 1964, he was accused of working to overthrow the government. The government increased Mandela’s sentence to life in prison.

MANDELA IN PRISON
The government sent Mandela to a prison on Robben Island, off the coast of South Africa. The prison conditions were harsh. Mandela was allowed only one visitor every six months. Every day he was forced to break rocks in the prison yard for many hours. During this time, Mandela became the world’s most famous political prisoner. Leaders around the word demanded Mandela’s freedom. They wanted apartheid in South Africa to end. In 1982, the government moved Mandela to a prison on the mainland. This was during a time of growing violence in South
Africa. Many people protested in the streets against apartheid.

PEACEMAKER
The government began secret talks with Mandela. They believed that if anyone could stop the trouble, Mandela could. He was a popular leader who had won the support of many South Africans. In 1990, Mandela was released after spending 27 years in prison.The government lifted the ban on the ANC. Mandela became its leader in 1992. Mandela soon began talks with the government aimed at ending apartheid. Many white people worried about giving blacks equal rights. Mandela worked with South Africa’s president, F. W. de Klerk, to promote peaceful relations between blacks and whites. For their efforts, Mandela and de Klerk won the Nobel Peace Prize in 1993.

PRESIDENT OF SOUTH AFRICA
In 1994, South Africa held elections. For the first time in South Africa’s history, men and women of all races could vote. Mandela became the first black president of South Africa. He brought an end to the hated apartheid system. After five years as president, Mandela retired from political office. He returned to live in the Transkei region, where he grew up.

Islam

2009-05-15T15:59:00.001-07:00

Islam is the second largest religion in the world. Only Christianity has more followers. People who follow the religion of Islam are called Muslims. Today, almost 1 billion people call themselves Muslims. Most Muslims live in a string of countries that extends from Morocco in North Africa to
Indonesia in Southeast Asia.

BIRTH OF ISLAM
An Arab trader named Muhammad was the founding prophet of Islam. He lived in Mecca, a busy trading town in Arabia (now called Saudi Arabia). Mecca had temples built to honor various pagan gods. Pilgrims visited these temples to worship statues of the gods. One day, while fasting in a cave, Muhammad had a vision. He returned to Mecca to preach a new religious message. He said there is only one god, not many, and no one should worship idols (statues of gods). He called on the people of Mecca to surrender themselves to Allah, as he called God.

ISLAM GAINS POWER
Muhammad’s message angered some Meccans. In the year 622, they forced him to flee to another city, now known as Medina. That journey—or Hegira, as Muslims call it—marks the beginning of the Islamic calendar. In Medina, Muhammad became the head of the community. Soon, he led his Muslim followers back to Mecca. After a battle, the Meccans accepted Islam. Within a century, Muslims ruled an empire that stretched from India to Spain. Throughout this empire, Islam took root. The empire crumbled after a few centuries, but many people of these lands remained Muslims.

ISLAM DIVIDES
After Muhammad’s death in 632, Muslims disagreed about how the next leader should be chosen. One group came to be known as Sunnis and the other as Shias. Islam remains divided into these two branches. By the year 900, a Muslim movement called Sufism had developed. Sufis seek a personal experience of God. Many great Muslim poets have been Sufis. Sufis helped to spread Islam long after the Islamic empire crumbled.

WHAT DO MUSLIMS BELIEVE?
The Muslim faith centers on five beliefs and practices. These are known as the five pillars of Islam. According to the five pillars, Muslims must
1. Accept that only one God exists and Muhammad was his messenger.
2. Perform certain prayers five times a day.
3. Fast from dawn to dusk during a month called Ramadan.
4. Give a portion of their wealth to the poor.
5. Visit Mecca at least once if they are able to. These practices are based on the holy book of Islam, the Qur’an (also spelled Koran). Muslims believe that the Qur’an is the word of God, delivered through Muhammad. Muslim practices are also based on the Hadith—Muhammad’s own deeds and sayings as eported by his companions.

ISLAMIC LAWS AND CUSTOMS
In the early Islamic empire, the government and the religion were he same. All questions that came up were decided by religious scholars. Their decisions filled in a set of laws called the Sharia. These laws, which are also drawn from the Qur’an and Hadith, cover every aspect of life. They tell what crimes should be punished and how. They set the rules for marriage, contracts, and inheritance. Islam is thus a complete way of life. In this way of life, men and women generally have different roles. Their activities are often kept quite separated. Families sometimes arrange marriages. Women are expected to be well covered when they go outdoors. In some countries, they have to wear a veil covering their body. In Arab countries, such a covering is called a burka. The mosque is the traditional place of worship for Muslims. Friday is their holy day, or day of worship. Islam teaches that all Muslims are equal before God. This teaching gives Muslims around the world a sense of community, no matter what country or social class they come from.

Amphibians

2009-05-15T09:14:00.000-07:00

Say you are thirsty. Can you put your hand in a glass of water and drink it through your skin? Of course you can’t! But some animals can absorb water this way. These animals are called amphibians. Amphibians are very interesting creatures. They live a kind of double life. They spend the first part of their life in water and the second part on land. In fact, the word amphibian
comes from two Greek words that mean “both” and “lives.”

KINDS OF AMPHIBIANS
An amphibian is an animal that has moist, hairless skin. Amphibians are cold-blooded, which means they cannot make their own body heat. They get warm in the sun and cool off in the shade. The three main groups of amphibians are frogs and toads, salamanders, and caecilians. All amphibians have backbones. The three kinds of amphibians look very different from each other.
Frogs and toads have legs but do not have tails. Salamanders have short legs and long bodies ending in tails. Caecilians do not have any legs. They look a lot like big earthworms. There are more than 4,000 different species (kinds) of amphibians in the world. Nearly all amphibian species are frogs or toads. Toads differ from frogs in the roughness of their skin. Toads have bumpy skin, and frogs have smooth skin. Toads also have shorter legs than frogs. The biggest amphibian found on Earth is the Japanese giant salamander. It can grow to be longer than 5 feet (1.5 meters). Tiny frogs, such as the gold frog, are only about 0.4 inch (about 1 centimeter) long.

THE DOUBLE LIFE
Most amphibians start out as larvae. Larvae look totally different from adult amphibians. Frogand toad larvae are sometimes called pollywogs or tadpoles. These larvae look more like fish. They live underwater and have a tail that they use for swimming. They breathe through gills to get oxygen from the water. An amphibian’s body changes completely when it goes from a larva to a grown-up. This kind of change is called metamorphosis. Most grown-up amphibians have lungs for breathing instead of gills. Some have both lungs and gills. Full-grown amphibians have legs for moving about on land. Frogs and toads have strong hind (back) legs for jumping. Most salamanders have four short legs and a long, strong tail. The tail helps them keep their balance while walking and pushes them forward while swimming. Caecilians never grow legs. They use their hard heads as battering rams when they burrow in the soil. In water, they swim as eels do, by wiggling their wormlike bodies back and forth. AMAZING SKIN Amphibians can “drink” through their skin. The skin of an adult amphibian is able to absorb water from its surroundings. Most amphibians do not even swallow water. Their skin soaks up as
much as they need. Amphibians also breathe through their skin. Their skin takes in oxygen as well as water. Most grown-up amphibians get oxygen through both their lungs and their skin. But some salamanders get all their oxygen through their skin. They do not even have lungs or gills. Slimy stuff called mucus covers an amphibian’s skin. The mucus keeps just the right amount of salt and water in the amphibian. In some species the mucus is poisonous, which helps keep predators from eating the amphibian. SENSING THE WORLD Some amphibians have good eyesight and hearing, and some do not. Most frogs and toads can hear well. Salamanders, caecilians, and some frogs can only sense vibrations in the ground or water. Frogs have great eyesight. They see through two bulging eyes that stick out from their heads. Most caecilians are totally blind. All amphibians can smell and taste pretty well. Caecilians use feelers on their heads. Amphibians have a place in their mouths called Jacobson’s organ that helps them smell and taste the world
around them.

AN AMPHIBIAN’S LIFE
Amphibians that live in cold places spend most of their time trying to keep wet and warm. In hot places, they try to keep wet and cool and usually come out only at night. During the day, theystay under rocks or logs or in the ground. Amphibians hibernate (become inactive) during cold winters. They become inactive in hot places during the summer. Almost all grown-up amphibians are meat eaters. Frogs and salamanders have sticky tongues. They flick out their tongues to catch insects, spiders, and other animals. Caecilians have sharp teeth for grabbing their prey. Amphibians mate when it is rainy outside. They gather in groups tofind mates. Some male salamanders show bright colors to get the attention of females. Male frogs call out to females. Their call sounds like this: “ribbet.” Female amphibians lay eggs in water or wet places on land. Larvae hatch out of the eggs. No one knows how long amphibians live in thewild. Some capturedtoads have lived 30 years.

WHERE AMPHIBIANS LIVE
Amphibians live everywhere except Antarctica. They can be found in grasslands, rain forests, evergreen forests, deserts, and mountain areas. Caecilians live only in the tropics. Amphibiansneed water to breed and have babies. So most of themlive near ponds, swamps, or streams. A few species can even find water in deserts. Some burrowing frogs live in a dry part of
Australia called the outback. They breed and feed only when it rains, which is not very often.

DISAPPEARING AMPHIBIANS
Amphibians have lived on Earth for more than 300 million years. But recently, something scary has been happening to them. Amphibians are disappearing. There are fewer and fewer amphibians in many parts of the world, including North America, South America, and Australia. No one knows why. Some amphibians have also been found with extra legs or other strange problems. Scientists are working hard to find out what is happening to the amphibians.

Flags

2009-05-15T09:04:00.000-07:00

Pieces of cloth in different colors, shapes, and patterns flap and snap in the breeze. They usually hang from poles or staffs. They stand as proud symbols for countries, states, persons, or groups. Sometimes, they are waved as a signal to people. What are these high-flying spectacles? Flags, of course!

NATIONAL FLAGS
The most important and best-known flags are national flags. A national flag stands for the people of a country. Every country has its own unique flag. The national flag of the United States is famous for its stars and stripes and colors of red, white, and blue. Most countries, including the United States and Canada, have a number of official flags. Some flags represent smaller areas of countries. Every state in the United States and every province in Canada has its own flag. Many cities have their own flags, too. Many countries create special flags for important government leaders. The U.S. president, vice president, and cabinet members all have unique flags.

MILITARY FLAGS
Most countries have special military flags. In the United States, different military units carry different flags. The U.S. Navy, Coast Guard, Army, Air Force, and Marine Corps all fly different flags. Navies fly flags on ships at sea. A national flag shows a ship’s nationality. Other flags are used for signaling, or sending messages, to other ships. Military units on land carry their own flags. Larger units, such as U.S. Army regiments, carry special flags, called colors. Smaller army units often fly their own flags, too. Hundreds of years ago, soldiers known as knights charged into battle beneath fluttering pennants. A pennant is a triangle-shaped flag. Today, many military units still carry pennants.

PERSONAL FLAGS
Long before there were national flags, kings and nobles flew their own flags. They decorated their flags with their family crest. The largest flag, called a standard, marked the presence of a king. Other nobles flew flags of different sizes and shapes based on their rank. As kings shaped their lands into nations, national flags grew in importance. Today, we recognize countries all around the world by their unique flags, called national standards. But most royal families still fly their own special flags.

Albert Einstein

2009-05-13T11:37:00.000-07:00

Dents in space, light in bundles, and matter that turns into energy sound like science-fiction fantasies. However, Albert Einstein said they were real. Other scientists proved through observations that Einstein’s theories were right. Einstein revolutionized the science of physics and helped bring in the atomic age.

WHERE DID EINSTEIN GROW UP?
Albert Einstein was born in Ulm, Germany, on March 14, 1879. He grew up in Germany, Italy, and Switzerland. Einstein taught himself geometry when he was 12 years old. School bored him because it required endless memorizing and reciting. He often skipped classes to study on his own or to play his violin. Yet he graduated from college in 1900 and earned a Ph.D. degree in 1905. From 1902 to 1907, Einstein worked as a clerk in the patent office in Zürich, Switzerland. His job left him plenty of time to think.

WHAT DID EINSTEIN THINK ABOUT?
Einstein thought about the rules that govern the way the world works. For example, he explained why small particles in liquids wiggle around, a movement called Brownian motion. He said that the particles were being bumped into by tiny bits of matter called atoms that are too small to see. He also thought about light and electricity. Einstein knew that light shining on metal sometimes causes electricity to flow. He explained this result, called the photoelectric effect, by saying that light is made of tiny bundles of energy called photons. Photons hitting the metal knock particles called electrons away. Since electricity is simply moving electrons, he had solved the mystery of the photoelectric effect. In 1921, Einstein won the most famous prize in science, the Nobel Prize, for this work. Another thing Einstein thought about was time. He said that time does not always flow at the same rate. He proposed that motion affects time. He calledthis idea the special theory of relativity. Einstein then came up with his general theory of relativity. This theory has a new explanation for gravity. Einstein said that gravity comes from curves or dents in the fabric of space. Objects make dents in space the way a bowling ball makes a dent in a mattress. The Moon falls into the dent made by Earth and rolls around the Earth. Scientists later proved that the dent a star makes in space-time bends light as the light passes by. Einstein changed physics by showing that new ideas could come just from thinking. Before Einstein, most new ideas in physics had come from experiments in the laboratory.

EINSTEIN AND ATOMIC ENERGY
Einstein also said that matter and energy are the same thing. He expressed this relation in a famous equation: E=mc2. This equation says that energy (E) equals mass (m) times the speed of light squared (c2). Energy can therefore be changed into matter, and matter into energy. The ability to turn matter into energy led to the development of the atomic bomb and nuclear power.

FAME AND LATER YEARS
Einstein’s theories made him famous, even though few people understood them. He became a university professor and director of a physics institute in Berlin, Germany. After the Nazis rose to power in Germany, Einstein left. In 1933, he came to the United States, where he lived the rest of his life. Einstein died in Princeton, New Jersey, on April 18, 1955. Einstein’s last great idea was that every force in nature is part of one master force. Physicists are still working on this idea, which they call the theory of everything.

Leonardo da Vinci

2009-05-13T11:23:00.001-07:00

Leonardo da Vinci excelled as a painter, sculptor, architect, engineer, and scientist. He had endless curiosity. Leonardo wanted to understand how things worked. He wanted to put down on paper what he saw. He left thousands of pages of drawings and notes that recorded his thoughts.

GOOD AT EVERYTHING
Leonardo was born in 1452 in the small town of Vinci, near Florence, Italy. He had little schooling and was largely self-taught. Leonardo seemed to be good at everything he tried. He was handsome, a good speaker, and a fine musician. He trained as a painter with Andrea del Verrocchio, a leading artist in Florence. Leonardo later worked for dukes and kings.

HIS MOST FAMOUS PAINTINGS
Leonardo produced a relatively small number of paintings, and he left some of them unfinished. But he had original ideas that influenced Italian artists long after his death. Leonardo believed painting was a science. He applied scientific thinking in his art so that his paintings looked more like the real world. One of his most important painting techniques was sfumato, a blending of one area of color into another so there are no sharp outlines. Leonardo used sfumato in one of his most famous paintings, the Mona Lisa. When you look at this portrait, notice how colors shade into each other on her face and hands. See how Leonardo has blurred the edges of her mouth to give her the hint of a smile. This mysterious smile has fascinated people for centuries. It looks as if Mona Lisa’s expression might change at any moment because of the way Leonardo has softened the edges of the mouth, eyes, and cheeks. She seems almost alive. Many people consider a mural by Leonardo known as The Last Supper to be his masterpiece. Christ, seated in the middle of The Last Supper, has just announced that one of his 12 apostles will betray him. Leonardo places the figures in this painting in a way that increases the drama of the announcement. Christ is the calm center. His body, which is set slightly apart from the others, forms a stable triangle. The apostles are arranged in four groups, some leaning toward Christ and some leaning away. Their gestures and the expressions on their faces reveal their reactions to Christ’s words.

HIS DRAWINGS AND NOTEBOOKS
Drawing was Leonardo’s favorite tool. He said that drawing was a better way of communicating ideas than words were. He drew catapults and war machines. He drew the muscles and skeletons of human beings and other animals. He drew clouds, swirling water, and storms. He designed churches that were never built. Leonardo’s drawings and theories are contained in numerous notebooks. His ideas were far in advance of what other people were thinking at the time. But the notebooks were not published during his lifetime. Had his notebooks been published, they might have revolutionized scientific thinking in the 1500s. Leonardo’s deep love of research was the key to both his artistic and scientific endeavors. Leonardo died in 1519.

Cold War

2009-05-13T11:16:00.000-07:00

People once thought the Cold War would never end. Sometimes they feared nuclear bombs would blow up the world. Now the Cold War has faded to a distant memory. The Cold War was a conflict primarily between the United States and the Soviet Union. Each power brought other countries into the conflict on its side. The Cold War lasted more than 40 years, from the mid-1940s to the end of the 1980s. Why was the war “cold”? Because the United States and the Soviet Union never got into armed combat—a shooting or “hot” war—with each other.

HOW DID THE COLD WAR BEGIN?
The Cold War began soon after World War II ended in 1945. The United States and the Soviet Union had been allies (friends) in defeating Nazi Germany in that war. The Soviet army had invaded Germany from the east. After the war, the Soviet Union kept control of countries in Eastern Europe that it had freed from German control. Those countries included Bulgaria, Czechoslovakia, Hungary, Poland, and Romania. An iron curtain fell across Europe. That’s how Winston Churchill, Britain’s leader during World War II, described the division of Europe. There was no actual curtain, but there were strong barriers between Soviet-controlled Eastern Europe and largely democratic Western Europe. The Soviet Union insisted on that. Barbed wire and armed soldiers at borders kept Eastern Europe separate from Western Europe during the Cold War. Defeated Germany was split into East Germany under Soviet control, and West Germany. Berlin, Germany’s former capital, was a divided city. In 1961, a concrete wall went up in Berlin, along the dividing line. Broken glass on top of the Berlin Wall kept people from going over it and escaping to West Germany.

WHAT WAS THE CONFLICT ABOUT?
During the Cold War, the Soviet Union and the United States had different political and economic systems. The Soviet system was called communism. The United States and its allies feared that the Soviet Union wanted to spread communism to the rest of the world. They disliked this system. The goal of communism was to end private ownership of property. Under communism, the people would own everything communally (jointly). They would make decisions as a group. But things didn’t work out that way in the Soviet Union. Instead, the country became a dictatorship under the control of the Communist Party. Many freedoms were lost. Millions of people who disagreed with the Communist Party were arrested and sent to prison camps. The United States and most of the countries of Western Europe haddemocratic governments rather than dictatorships. They valued freedoms that had been lost in the Soviet Union. Their economic system—capitalism—was based on private ownership and on individual rather than group effort. At times during the Cold War, the United States lost some freedoms because of fear of communism. During the 1950s, many Americans who were suspected of sympathy for communism lost their jobs.

WHAT WAS THE ARMS RACE?
During the Cold War, each side built up its arsenal of weapons, especially nuclear weapons—atomic bombs and hydrogen bombs. These weapons could destroy cities and kill hundreds of thousands of people. Each side was afraid the other would start a nuclear war first. A nuclear arms race began. Each side said it needed more weapons to retaliate (fight back)in case of a nuclear attack. Both sides believed that if they had enough weapons the other side wouldn't dare start a nuclear war. If it did, it would face total destruction in retaliation.

HOW DID THE COLD WAR SPREAD?
The Soviet Union had put Communist governments in power in Eastern Europe. Communist leaders had come to power in China. The Soviet Union and China backed groups that tried to take power in other countries. Wars broke out. The United States entered wars to stop the spread of communism. With the Korean War (1950-1953), the Cold War reached Asia. China backed North Korea, and the United States supported South Korea. Korea was still divided when fighting stopped in 1953, and it remains divided today. In 1962, the Cold War brought the world to the edge of nuclear war. The United States discovered that the Soviet Union had installed nuclear missiles in Cuba. The missiles were pointed at the United States. War seemed certain, but the Soviet Union backed down and removed the missiles.

The United States fought the spread of communism in the Vietnam War (1959-1975). The U.S. government feared that if one Asian country fell to communism, the rest of Asia would become communist as well. The war ended with a communist victory in Vietnam. But communism did not spread throughout Asia.

HOW DID THE COLD WAR END?
The Soviet Union nearly went broke as a result of the Cold War weapons race. The Soviet people had given up hope of a better life ahead. In the mid-1980s, a new leader came to power in the Soviet Union, Mikhail Gorbachev. Gorbachev was determined to improve life in his country. In the process, he helped bring the Cold War to an end. Gorbachev called for more freedom for his people. He and U.S. resident Ronald Reagan agreed to destroy many of the weapons their countries had built. Gorbachev encouraged change in Eastern Europe, and he said that Soviet troops would no longer keep communist governments in power there. One after another, the countries of Eastern Europe got rid of their communist leaders. In 1989, the Berlin Wall came down. Europe was no longer divided. Finally, parts of the Soviet Union declared their independence. The Cold War ended with the collapse of communism in Eastern Europe in 1989, and the breakup of the Soviet Union in 1991.

Architecture

2009-05-13T11:07:00.001-07:00

We ask for a lot from our buildings. They have to keep out the rain, wind, rats, and bugs, not to mention our enemies. They need to store our stuff, keep us warm when it’s cold, and keep us cool when it’s too hot. We also like our buildings to be well constructed and beautiful. Architecture is the important art and science of making buildings. Architects are the bosses who design buildings and supervise their construction.

WHAT ARE BUILDINGS MADE OF?
Buildings once were made of whatever materials were available. Where forests grew, for example, people used wood. Where there were no trees, people used the earth itself. They dried mud in the sun to make bricks. In the far north, they used blocks of snow or ice. These materials aren’t long-lasting, however. Mud bricks wear away. Wooden buildings catch fire and burn down.
Stone won’t catch fire, and it can be expected to endure. Many kinds of stone, including marble and limestone, are good materials to build with. Steel is strong and lightweight. It lets architects build tall buildings, like skyscrapers. But deciding on what materials to use isn’t usually the first thing an architect thinks about. The architect needs to know the reason for the building. How will the building be used?

REASONS FOR BUILDINGS
Shelter is the most basic reason for building. Buildings shelter us at home, at work, and at play. All buildings must shelter us from rain, wind, sun, and cold. Buildings also provide security. Some of the world’s most permanent structures were built to defend against enemies. Castles had moats and drawbridges to keep enemies out, and high walls from which to pour down boiling oil on them. Some of the most impressive buildings provide places for worship. Temples, churches, and mosques must meet spiritual needs. The soaring heights of a Gothic cathedral, for example, inspire amazement and admiration. Some buildings are just for showing off. Kings and emperors insisted on grand palaces and castles. People with money have always demanded that architecture display their wealth. Today, large corporations, governments, and universities demonstrate their importance by putting up impressive buildings. Architects today design all kinds of buildings. Our way of life calls for office buildings, large apartment complexes, shopping centers, schools, hospitals, airports, and hotels.

LANDMARK ARCHITECTURE
We judge the beauty of architecture in various ways. Some buildings have been judged especially great. In the past, architects lavished attention on religious structures. Today, many of the most
exciting buildings are museums and houses. Many people think the Greek temples are the most beautiful shrines of all time. The Greeks put columns around the outside of their temples. One masterpiece is the Parthenon, a temple on a hilltop in Athens, Greece. We admire its simplicity, but the carvings on the temple were once painted bright colors. Saint Peter’s in Vatican City was a major project of the 1500s. With its dome by Michelangelo, Saint Peter’s influenced many state capitol buildings in the United States.

America’s most famous architect is Frank Lloyd Wright. A number of his buildings are national landmarks. Most famous are the Guggenheim Museum in New York City and Fallingwater, a house in Pennsylvania. The Guggenheim’s spiraling ramp provides a dramatic showcase for artwork. Fallingwater overlooks a stream and waterfall. Its terraces appear to project from the surrounding hillside. Frank Gehry designs buildings that combine many irregular shapes. His Guggenheim Museum in Bilbao, Spain, uses titanium metal to create a curving, lightweight surface that shimmers in sunlight.

Amazon River

2009-05-13T10:50:00.000-07:00

A dense, green forest lines the riverbanks. Monkeys chatter in the trees. Off to the side, a big crocodile sticks its eyes and nose out of the water. This is what a boat trip on the Amazon River can be like. Be careful not to fall overboard! Fish called piranhas may be swimming near the boat. A group of piranhas can gobble up a large animal in minutes.

THE MIGHTY AMAZON
The Amazon is a long river in South America. The river starts in snow and tiny streams, high in the Andes Mountains in Peru. It flows east through Brazil. After 4,000 miles (6,400 kilometers), the Amazon empties into the Atlantic Ocean. Along its route, hundreds of streams and smaller rivers empty into the Amazon. As a result, the Amazon carries more water than any other river in the world. Although the Amazon is the largest river in the world, it is not the longest. Only the Nile River in Africa is longer than the Amazon.

The Amazon changes size through the year. It is biggest from January to June, when heavy rains fall in Brazil. During the rainy season, the river is more than 50 miles (80 kilometers) wide in some places. The Amazon dumps several million tons of mud, sand, and other sediment in the Atlantic Ocean every day. Much of this sediment has washed down from the Andes Mountains. It turns the Amazon a muddy yellowish color. The sediment changes the color of the Atlantic for about 200 miles (about 320 kilometers) from the mouth of the Amazon.

WHAT ANIMALS LIVE IN THE AMAZON?
The Amazon is home to many interesting animals. The piranha is an Amazon fish with a bad reputation. One species (kind) of piranha has powerful jaws and sharp teeth that can tear flesh from bones. However, most piranhas eat plants. Stingrays and electric eels live in the Amazon, too. Stingrays have poisonous stingers on their tails. Snakelike electric eels use their electricity to stun prey. Giant otters, river dolphins, and manatees are among the mammals found in the Amazon. Crocodiles called caimans and giant turtles also live in the river.

THE AMAZON RAIN FOREST
A vast tropical rain forest lies next to the Amazon River in Brazil and neighboring countries. More than seven times the size of Texas, it is the largest rain forest in the world. The Amazon rain forest is home to colorful scarlet macaws, stealthy jaguars, noisy howler monkeys, bloodsucking vampire bats, three-toed sloths, long-nosed tapirs, and powerful anaconda snakes.
Many useful plants grow in the Amazon rain forest. They provide food, building materials, rubber, medicines, and other products.

X Rays

2009-04-26T19:21:00.000-07:00

Imagine that you could see right through your own skin. You could see the bones inside your body. You could watch food go down your throat when you swallow it. Imagine looking inside someone’s suitcase to see what’s inside. Does that sound impossible? Not when you know about X rays!

WHAT ARE X RAYS?
X rays are very powerful light rays that your eyes can’t detect. These light rays can slip through objects that visible light bounces off. We use X rays as a powerful tool to detect and discover things our eyes can’t see.

HOW WERE X RAYS DISCOVERED?
X rays were discovered by accident. In 1895, a man named Wilhelm Roentgen was experimenting with electricity in vacuum tubes in a black cardboard box. He noticed that a special screen he had nearby glowed when electricity went through the tubes. He experimented more and determined that invisible light rays from the tubes caused the screen to glow. These rays went right through the cardboard box! He named the invisible light rays he had found X rays.

WHAT ARE X RAYS USED FOR?
Just a few years after X rays were discovered, doctors were already using them to find bullets inside people who had been shot. Doctors later began to use X rays to find out if people are sick or have broken bones. Dentists use X rays to check up on people’s teeth. An X-ray device called a CAT scan rotates around a person and creates a 3-D picture of the person’s insides on a screen. This device gives doctors clear views inside any part of the person’s body. Scientists who study matter and energy often use X rays in their research. X rays help them see what things are made of. Many chemical elements were discovered using X rays. Industries use X rays to test products and materials for flaws such as cracks in an airplane wing. X rays are also used to tell whether gems and works of art are real or fake. Border guards use X rays to look inside cars and containers. The X rays can find goods that are being smuggled from one country to another. Airports use low energy X rays to see inside luggage and check for dangerous items.

DOCTORS’ X RAYS
When a doctor takes an X ray of you, the X-ray machine shoots X rays at you. Most of the rays go through you and into a special film, which catches them. Some of the X rays that hit your bones, however, don’t make it through you. Bones absorb X rays more than other parts of your body. Because X rays absorbed by your bones never make it to the film, lighter areas appear on the film where your bones are! These lighter areas provide a picture of the bones. X rays can be harmful. Doctors use X rays to kill cells that are harmful to people, such as cancer tumors. Because too many X rays can be harmful, doctors warn that X rays should be used only
when necessary.

Ships

2009-04-26T19:15:00.000-07:00

Every day, huge ships made of steel cross the oceans and travel the world’s great rivers and lakes. Powerful engines turn propellers that make the ships go. Ships transport people and goods to all parts of the world. Ships are very important to the way we live. Ships carry oil that is made into gasoline for our cars. They bring in much ofthe food we eat and the clothes we wear. They carry computers, furniture, and televisions for our homes. Look around you. Many of the things you see traveled to where you are on a ship.

THE PARTS OF A SHIP
Ships may look very different from each other, but they all have the same basic parts. All ships float in water. The part that floats is called the hull. Inside the hull there are decks. Decks are like the floors in a building. You can go up and down from one deck to another.

HOW SHIPS MOVE THROUGH WATER
The front of a ship is called the bow. The back is called the stern. Attached to the stern is a wooden or metal plate called the rudder. A steering wheel or a stick called a tiller makes the rudder swing back and forth. Moving the rudder makes the ship turn. Some ships use sails to move. Sails are big sheets of fabric. The sails hang from a long pole called a mast. Ships with sails use the energy of blowing wind to move through the water. Most modern ships have engines that burn fuel. Engines make power to turn propellers at the stern. Propellers make ships go
through the water.

THE AGE OF SAILING SHIPS
By about 5,000 years ago, the Egyptians were building some of the first sailing ships. They made them by tying bundles of reeds to a wooden frame. The ships carried cargo and had one or two square sails. The best ancient shipbuilders were the Phoenicians. They made cargo ships and warships called galleys. Galleys had sails and many oars. The ancient Greeks fought with the Phoenicians. The Greeks added a big spike to the front of their galleys. They used the spike to ram into Phoenician ships. In China and other parts of Asia, builders made cargo ships called junks. Junks had a flat bottom, a square bow, and a rudder. The sails had pieces of bamboo in them to make them stiffer. Arab builders began to use triangular sails called lateens. A ship with lateen sails could sail almost directly into the wind. In the 1200s, Europeans began building ships with three masts and many square and triangular sails. These ships were called full-rigged ships, or square-riggers. Starting in the 1400s, European explorers set off on voyages in these ships to faraway parts of the world. Christopher Columbus, Vasco da Gama, and other explorers used square-rigged ships.

In the 1600s, the Spanish built huge ships called galleons. In the 1700s and 1800s, the British built big sailing ships that they used to fight sea battles. The fastest sailing cargo ships were the clipper ships of the mid-1800s. They had sleek, narrow hulls and as many as six sails on each tall mast. MODERN SHIPS During the 1800s, iron and steel hulls replaced wooden hulls. New types of engines were also developed. For the first time, ships could move without wind or human-powered oars. Steam engines fueled by coal replaced sails. Later, engines that used oil as a fuel replaced steam engines. Today, most ships have steel hulls and are driven by powerful motors that turn big propellers.

CARGO SHIPS
There are many kinds of cargo ships. Container ships carry cargo in huge boxes the size of railroad cars. Oil tankers and supertankers carry oil in their hulls. Freighters transport tons of coal, grain, and ore.

PASSENGER SHIPS
There were no passenger ships in ancient times. Travelers had to look for space on a cargo ship. Most passengers slept wherever they could find a spot on the deck. After Europeans learned about the Americas and Australia, settlers wanted to move to these new lands. Full-rigged ships carried passengers along with cargo. It was not very comfortable traveling on those wooden sailing ships.

By the mid-1800s, shipping companies began to offer regular passenger service. Companies competed with each other for passengers. They built luxurious ocean liners that could cross the
Atlantic Ocean in just a few days. In the 1950s, airplanes became more popular than ships for traveling over oceans. Today, most passenger ships are cruise ships. You can take a vacation aboard big cruise ships.

NAVY SHIPS
For many years, battleships were the biggest warships. They were used in World War I and World War II. Today, aircraft carriers are the biggest warships. The largest carriers can hold 85 airplanes. They have crews of more than 5,500 people. Modern navies have many other kinds of ships. Submarines are ships that can dive underwater. Some submarines carry missiles toattack enemy ships. Cruisers escort and defend aircraft carriers from attack by planes and submarines. Destroyers defend carriers and merchant ships from air and submarine attacks. Frigates escort
and defend ships from submarines.

THE NEWEST SHIPS
Shipbuilders are looking for ways to build big ships that go faster and carry more cargo. They are looking for new hull shapes that go faster in the water. They are also looking for better engines. Water jet engines may replace propellers. A jet boat engine works by shooting out water, just as a jet plane engine shoots out air.

Muscles

2009-04-26T19:11:00.000-07:00

How strong are you? Can your legs run fast? Can your arms lift heavy books? Your muscles make your legs run fast. Your muscles let your arms lift heavy books. Without muscles, you wouldn’t be able to move at all!

HOW DO MUSCLES WORK?
Muscles work by tightening and loosening. Tightening is called contraction. Loosening is called relaxing. Your nerves tell muscles when to contract. Suppose you see a cookie on a table. You want to pick up that cookie and eat it. Your brain sends out a signal. Nerves carry the signal from your brain to your arm and hand muscles. The signal tells muscles in your arm to contract. Your arm reaches out for the cookie. Then the signal tells muscles in your hand to contract and grab the cookie. Muscles in your arm contract to bring the cookie to your mouth. Your hand pops the cookie into your mouth. Your jaw muscles contract and relax so you can chew the cookie. Yum.

WHAT ARE MUSCLES MADE OF?
Muscles are bundles of thin strands called fibers. The muscle fibers are made of substances called proteins. There are two types of muscle fibers. The two types are slow twitch and fast twitch. Your fast-twitch muscle fibers contract rapidly. These fibers give you bursts of power. When you suddenly jump or run fast while playing tag, your fast-twitch muscle fibers are hard
at work. Slow-twitch muscle fibers allow you to keep doing exercises. They give you endurance. When you run a long way, your slow-twitch muscle fibers are doing most of the work. Some kinds of muscle have both fast-twitch and slow-twitch fibers.

ARE THERE DIFFERENT KINDS OF MUSCLES?
There are three types of muscle called skeletal, smooth, and cardiac. Each kind of muscle has a different job to do. Your skeletal muscles are attached right to your bones. These muscles contract and relax to make your bones move. These are the kind of muscles you use to run or swim or reach for cookies. You usually have to think about making these muscles contract. Smooth muscles move automatically. You do not have to think about your smooth muscles to make them work. Your digestive system is surrounded by smooth muscle. Smooth muscles push
food through your digestive system. Your biggest blood vessels are surrounded by smooth muscle to make them stronger. Your cardiac muscle makes your heart beat. You do not have to
think about moving your cardiac muscle. Your cardiac muscle contracts automatically.

WHAT MAKES MUSCLES STRONG?
Exercise makes muscles stronger. Lifting heavier and heavier weights makes your skeletal muscles stronger. Many people lift weights to make their arm and leg muscles stronger. A gym
teacher or special trainer can show you how to lift weights safely. Running, walking, swimming, and jumping rope are exercises that can make your heart muscle stronger. Any exercise that makes your heart beat faster makes your heart muscle stronger. Muscles can get weaker if they are not used. Using your muscles every day keeps them strong and healthy.

Helicopters

2009-04-26T19:07:00.000-07:00

Helicopters can fly straight up. They can fly forward, sideways, and backward. They can even hover in one place. An airplane must speed down a long runway to take off and land. Wings hold an airplane in the air. Helicopters do not need runways, and they do not have wings.

WHAT HOLDS A HELICOPTER IN THE AIR?
Big blades on top of a helicopter keep it in the air. The blades are a little like fan blades. The blades spin very fast. Wind blowing down from the whirling blades holds a helicopter up. The blades also control the direction in which the helicopter flies. The blades make a loud chop-chop-chop noise as they turn. The noise caused people to nickname helicopters “choppers.” Helicopters cannot fly as fast as an airplane. The fastest helicopters go about 200 miles per hour (320 kilometers per hour). They also cannot go as far as an airplane. Helicopters burn a lot of fuel.

HOW MANY BLADES DO HELICOPTERS HAVE?
The blades of a helicopter are called the rotor. Some rotors have two blades. Some rotors have three or four blades. Some big helicopters have rotors with eight blades. Big helicopters sometimes have two rotors on top. A long metal tail sticks out from the back of most helicopters.
These helicopters have a small rotor on the tail. The tail rotor blows air sideways instead of down. It helps the helicopter steer.

HOW DO YOU FLY A HELICOPTER?
A helicopter has a cockpit just like an airplane. The controls are in the cockpit. A helicopter has two control sticks, or levers. It has two pedals on the floor. When you’re flying a helicopter, you push or pull on the stick on your left side to make the blades tilt. Tilting the blades makes the helicopter go up or down. A grip on this stick controls the speed. You twist the grip to make the helicopter go faster or slower. The other lever is between your knees. You move this stick around to make the helicopter fly forward, backward, or sideways. The pedals on the floor control the tail rotor. You step on the pedals to turn the helicopter. Pushing the left pedal makes the helicopter turn left. Pushing the right pedal makes the helicopter turn right. HOW DO WE

USE HELICOPTERS?
Helicopters can go places that are hard to reach. They can go places where airplanes cannot land, or where there are no roads for cars or trucks. Helicopters can rush injured people from a car accident to a hospital. They can rescue people from the tops of burning buildings. They can pluck them from trees in the middle of raging floods. They can lift people from the decks of sinking ships at sea. Military helicopters are important in war. They carry troops to battle. They carry wounded soldiers to hospitals. They can even shoot missiles. Helicopters do other kinds of work. Reporters can fly in helicopters to cover news stories. Police use helicopters to chase suspected criminals. Farmers can use helicopters to spray their fields. Sometimes helicopters work on construction. The carry heavy parts to the tops of buildings.

WHO INVENTED THE HELICOPTER?
People imagined machines like helicopters hundreds of years ago. The ancient Chinese made a spinning top that could rise up into the air. The Italian artist and inventor Leonardo da Vinci drew a design for a machine like a helicopter in 1480. The first helicopters flew in France in 1907. They were hard to control. People kept trying to build better helicopters. Finally, a Russian-born American engineer named Igor Sikorsky made a workable helicopter. It was the first helicopter with a tail rotor. He flew this helicopter in 1939. Most helicopters today are like the helicopter that Sikorsky built. Inventors and engineers are still working to make bigger and better helicopters.

Gold

2009-04-26T19:02:00.000-07:00

“Gold! Gold! Gold!” screamed the newspaper headlines. “Gold discovered in California!” That was in 1848. The news brought 100,000 people rushing to California. They came seeking the yellow metal that could make them rich. People since ancient times have used gold for jewelry and money. They used it in religious objects and works of art. Wars have been fought over gold.And sometimes, as in California, gold changed the course of history.

WHY IS GOLD VALUABLE?
Gold is unusual among metals. It does not rust or tarnish (grow dull and discolored). Gold coins recovered from sunken treasures are still as shiny as when they sank. Gold is soft enough to be easily shaped into jewelry and other items. An ounce (31 grams) of gold can be hammered into a sheet 16 feet (5 meters) on each side. It can be stretched into a wire 62 miles (100 kilometers) long. People find gold beautiful. And it is rare. All the gold in the world would fit in a cube 65 feet (20 meters) on each side. Because it is so rare, its value doesn’t change much from one year to the next. In ancient times, people could easily carry a lot of wealth in the form of a small bag of gold.

MINING GOLD
The easiest way to mine gold is with a pan. You fill the pan with sand or gravel that contains tiny bits of gold. Then you swirl the pan under a gentle stream of water. The lighter gravel or sand gradually washes out with the water. The heavier gold particles collect at the bottom of the pan. Gold is so heavy that it doesn’t take many tiny flakes to make an ounce. Today, gold is most often mined by digging underground with machines. Rock that contains gold is treated with chemicals to separate out the gold. Nuggets of solid gold are quite rare. The largest nugget ever found weighed about 130 pounds (59 kilograms). It was found in Australia in 1869. About two-thirds of all gold mined today comes from South Africa.

HOW IS GOLD USED?
Gold is used for many things besides coins, decorations, and jewelry. Gold conducts electricity very well. It is used in tiny electrical circuits. There are very small amounts of gold in your computer. Gold is also used to protect tall buildings and spaceships from the Sun’s heat. The Sun's rays bounce off even a thin coating of gold.Gold-coated mirrors are used in telescopes. Dentists use gold for tooth fillings. Gold is even used in medicine, to treat cancer and
arthritis.

GREEDY FOR GOLD
Gold has always made people who controlled it wealthy. Folktales of many peoples tell of greed for gold. The ancient tale of King Midas tells of a greedy king. Midas wished that everything he touched would turn to gold. But he was sorry when his wish was granted. He could not eat because his food and water also turned to gold. In the 1500s, the Spanish conquered Mexico and Peru while searching for gold. They brought back tons of gold looted from native peoples. The peoples of those regions had never considered gold very valuable. The discovery of gold in parts of western America and Australia brought in thousands of people seeking quick fortunes. Many stayed on and settled those regions.

Photosynthesis

2009-04-26T18:59:00.000-07:00

Plants need sunlight. Houseplants lean toward the Sun, and if they do not get enough light they wither and die. Plants use sunlight to make their food. This process is called photosynthesis.
Photosynthesis is a scientific word made up from Greek words. These words mean “putting things together using light.” Inside plants’ leaves, light causes air and water to combine to make new chemicals. These chemicals are food for the plants.

FOOD FACTORIES
In most plants, photosynthesis takes place mainly in the leaves. Like other living things, plants are made up of tiny cells. The cells in a plant’s leaves contain even smaller, disc-shaped parts called chloroplasts. Chloroplasts are the food factories where photosynthesis happens. A leaf the size of your little fingernail contains more than 10 million of them. Chloroplasts contain a chemical called chlorophyll, which is bright green. Chlorophyll gives plants their green color and makes photosynthesis work.

THREE INGREDIENTS
For photosynthesis to work, the chloroplasts need to collect three ingredients: sunlight, air, and water. Sunlight shines on the leaf, and the green chlorophyll inside the chloroplasts soaks it up. Air enters the leaf through tiny holes in the leaf’s surface, called stomata. Water is sucked from the ground by the plant’s roots. It travels through tubes in the stem or trunk to the leaves. When all three ingredients are present inside the chloroplasts, a chemical reaction takes place. The reaction takes place between a gas in air called carbon dioxide and hydrogen, a part of water. Sunlight causes these two to combine and make new chemicals called carbohydrates. Carbohydrates are plant food. Plants use these chemicals to live and grow.

THE KEY TO LIFE
Photosynthesis is not just important for plants—it is the key to lifefor all of us. Plants use photosynthesis to make food. We eat the leaves, roots, fruits, and seeds of plants. Spinach and lettuce are leaves. Potatoes and carrots are roots. Tomatoes and apples are fruits. Nuts are seeds. If you eat beef, lamb, or other meat, you still depend on plants. Meat comes from cows, sheep, and other animals that feed on plants. In this way, energy from the Sun is passed on through all the different living things on Earth. If it were not for photosynthesis, plants would not grow. There would be nothing for animals to eat, so they would not exist either.

BREATHABLE AIR
Photosynthesis also produces a gas, oxygen. Plants release oxygen into the air. Humans and other animals need oxygen to live. We breathe in the oxygen produced by plants during photosynthesis.

Eyes and Vision

2009-04-26T18:52:00.000-07:00

Think about all the things you do with your eyes. You watch TV and read books. You surf the Internet. You keep your eye on the ball when you play sports. You see your family and friends. Your eyes are your windows on the world. Your eyes are like cameras that focus on what is in front of you. Your eyes work together with your brain to create a picture of the world. The process of creating the picture starts when light rays enter your eyeball.

LOOKING AT YOUR EYEBALL
Look at one of your eyes in a mirror. Your eyeball is round. The inside of your eye is a transparent (see-through), jellylike material called the vitreous humor. The vitreous humor gives your eyeball its shape. You can’t see much of the vitreous humor because it is surrounded by an outer part, or wall. The wall of your eyeball is made up of outer, middle, and inner layers.

THE OUTER WALL
The outside layer of your eye is a protective coating called the sclera and the cornea. The sclera is the white part of your eye. The cornea is clear and goes over the center of your eye, the part you look through. Light rays enter your eye through your cornea.

THE MIDDLE WALL
The middle layer of the wall has three parts called the iris, the ciliary body, and the choroid. What color are your eyes? The color comes from your iris. Your iris is in the center of your eye. It can be shades of brown or blue. The black circle in the center of your iris is called the pupil. It gets bigger or smaller to control how much light comes into your eye. The ciliary body goes around your iris and connects to the lens of your eye. Muscles in the ciliary body pull on the lens to focus it. The blood vessels that bring blood to your eye are also part of the middle wall. These blood vessels are called the choroid.

THE INNER WALL
The inside layer of your eyeball wall is called the retina. Your cornea and lens focus light rays on your retina just as a camera lens focuses light rays on film. They bend light rays coming into your eye so that they will strike the center of the retina called the macula lutea. This is where you have your sharpest vision.

RODS AND CONES
Your retina has millions of light-sensitive cells called rods and cones. These cells pick up the tiniest dot of light that gets to your retina. There are bits of colored material called pigment in the rods and cones. Pigment in the rods lets you see shades of gray and helps you see at night. Pigment in the cones lets you see colors. The rods and cones change light rays into electrical signals. Nerves pick up these signals and carry them to your optic nerve.

OPTIC NERVES
An optic nerve leads from each of your eyes to your brain. Each eye picks up slightly different images. When these images get put together, you can see in 3-D. You have depth perception that lets you tell how far away things are. Your optic nerves are like big cables that carry all the signals to a special part of your brain. This “media center” in your brain makes a picture of the world. It gives you sight.

EYE PROTECTION
Your eyeballs are set into two holes in your skull called eye sockets. The bones of your skull protect your eyes. Muscles let you turn your eyes in their sockets. Eyelids and eyelashes also protect your eyes. You can close your eyelids to keep dust or bright light out. Your eyelashes are a fringe of short hairs on each eyelid. They screen out dust when your eyelids are partly closed.
Inside the eyelid is a thin layer called the conjunctiva. It covers part of the sclera. Each eye also has a tear gland that gives off salty liquid to wash small particles out of your eye.

VISION PROBLEMS
Do you wear contacts or glasses? If not, you probably know someone who does. Many people need contacts or glasses becausethey are nearsighted. Things far away look blurry. Light rays focus in front of the retina because the eyeball is too long. Some people have the opposite problem. They are farsighted and can’t see close-up things very well. In farsightedness, light rays focus behind the retina because the eyeball is too short. Astigmatism is another vision problem. A person has an astigmatism when their cornea is unevenly curved. Older people sometimes need reading glasses because the muscles in their eyes can no longer focus on things that are nearby.

EYE DISEASES
Diseases strike different parts of the eye. A sty is an infection of the eyelid. Conjunctivitis, or pinkeye, is an infection of the thin layers covering the eyelid and outer eyeball. Many eye diseases are most common in older people. Sometimes the lens of the eye gets cloudy over time. This condition is called cataracts. Retinas can get detached (come loose) from the back of the eye and cause blindness. Glaucoma occurs when fluid gets trapped between the cornea and the lens and puts pressure on the eye. A problem called macular degeneration affects the center of the retina. It can cause blindness in older people. There are many treatments for eye diseases.
Doctors treat infections with drugs. They use laser beams to weld detached retinas back into place. Surgeons can replace clouded lenses with clear plastic ones. They can also replace diseased corneas. It is very important to protect your eyes. Get regular eye examinations. Wear eye protectors when doing dangerous work or playing rough sports. Wear sunglasses that protect against harmful rays from the Sun. Your eyes are too important to take chances
with!

Communism

2009-04-26T18:48:00.000-07:00

What if nobody was rich and nobody was poor? Suppose valuable things like land and factories belonged to everyone. Imagine a world in which everyone worked, but no one got paid. Why? Because everything would be free. These are among the basic ideas of a movement called communism.

HOW WAS COMMUNISM BORN?
The ideas of communism became popular in the early 1800s. At that time, the first big factories were forming. Most workers in these factories earned low wages. Meanwhile, the factory owners were getting rich. This made many workers angry. Two Germans, Karl Marx and Friedrich Engels, turned this anger into a political movement. They announced the goals of this movement in their 1848 book, The Communist Manifesto. Marx and Engels said that all through history, different classes (groups of people) had been at war. They said the warring classes of their time were the owners of businesses and the people who worked for them. Marx and Engels called for a world in which the workers themselves owned all businesses. They said that if workers were in charge, everybody would get what they needed to live good lives. Marx explained these ideas further in a book called Das Kapital.

THE SPREAD OF COMMUNISM
The ideas of Marx and Engels spread through Europe. In Russia, a man named Vladimir Lenin said workers needed a small, organized group to lead a revolution on their behalf. To do this, he formed the Bolshevik Party (later called the Communist Party). In 1917, a revolution overthrew the emperor of Russia. In the fighting that followed, Lenin’s party took over. Soon, his communist government owned all the land, factories, stores, and businesses in Russia. The communists in Russia conquered some neighboring countries, too. They called their new empire the Union of Soviet Socialist Republics (USSR), or Soviet Union for short. After Lenin died, Joseph Stalin took power in the USSR. He used brutal force to make the Soviet Union into a communist country. Stalin killed, imprisoned, and tortured millions of people. He built a country in which everyone worked for the government. The government made almost all decisions for the people.

HOW DID COMMUNISM SPREAD?
In World War II, the Soviet Union fought on the side that won. When the war ended in 1945, the Soviet Union gained control over most of the countries in Eastern Europe. In China, meanwhile, a communist leader named Mao Zedong was rising in power. He seized control of his country in 1949. China, with its huge population, became the biggest communist country of all.

WHY DID COMMUNISM LOSE STRENGTH?
Many noncommunist countries fought to keep communism from spreading. They feared that communism would keep expanding and threaten noncommunist countries.The United States led the fight against communism, while the Soviet Union supported communist movements around the world. This struggle is known as the Cold War. It lasted for more than 40 years. By the 1980s, communism was failing. Under communism, the Soviet Union could not produce enough goods for its people. It grew poor. Eastern European countries began to break away from
the Soviet Union. In 1991, the Soviet Union itself broke apart into 15 separate countries. None of these countries have communist governments today.
In China, the Communist Party still holds power. But China is allowing privately owned businesses to grow again. Other communist countries today include Cuba, Vietnam, Laos, and North Korea. As a world movement, communism appears to be losing its
appeal.

Color

2009-04-26T18:43:00.000-07:00

The world is full of beautiful color. You see a violet dress, a blue car, a yellow flower, and a green tree. The colored leaves on trees in autumn mean that winter is coming. All kinds of colors are everywhere.

COLOR COMES FROM LIGHT
White light, including sunlight and light from a light bulb, is actually made of all the colors of the rainbow. Have you ever seen sunlight that hits a piece of crystal? Rays of blue, purple, orange, yellow, and red seem to shoot out from the crystal in all directions. The crystal spreads the colors of light apart a bit so you can see them separately. Scientists show the colors of light in a bar called a spectrum. A rainbow is a spectrum. Its colors go from red through orange, yellow, green, blue, indigo, and violet. Violet is purple and indigo is a deep purplish-blue. It’s easy to remember the colors of the spectrum because the first letter of each color makes up a name:
Roy G. Biv.

HOW DO WE SEE COLOR?
White light, such as sunlight or light from a light bulb, lets you see things. Dresses, cars, and all other things we see absorb (soak up) some of the colors of white light. The colors that don’t get absorbed bounce off of things. Red light bounces off a red dress. The dress soaks up other colors. Your eyes see the red light but not the other colors. Your eyes send this message to your brain. Your brain tells you that you are seeing a dress that looks red. Something very special happens when you see a red dress or green grass. Objects themselves don’t actually have color. What they have is the ability to reflect (bounce back) different types of light. When this reflected light enters your eyes, your brain interprets the different types of light as color. Your eyes and brain work together to translate the different types of light into different colors. COLORS OF PAINT Artists call three special colors the primary colors of paint. These colors are magenta (purplish-pink), yellow, and cyan (light greenish-blue). You can make other colors of paint by mixing the primary colors together. But you cannot mix other colors of paint to make a primary color. Suppose you want to paint a picture of an apple tree. You can make whatever colors you want to use with just four jars of paint: magenta, yellow, cyan, and white. You mix yellow and cyan to make green paint for the leaves. For the apples, mix magenta and yellow paint to make the color red. To paint the sky light blue, you must use some white paint. White makes other colors lighter. Mix magenta and cyan to make a deep blue. Then add some white paint to the blue paint until the blue becomes light enough for the sky. White paint mixed with blue or another color is called a tint. A light blue tint will make a color like a clear sky. If you mix all three colors together you get black paint. You can make a color darker by mixing it with black paint. Colors mixed with black paint are called shades. When you mix black and white
together, you get gray. By mixing together different amounts of magenta, yellow, cyan, and white paint, you can make lots of different colors. Everything you need to make beautiful paintings.

Cameras

2009-04-26T18:34:00.000-07:00

Can you imagine a world without cameras? There would be no photographs in newspapers, books, and magazines, or even on your computer. There would be no school pictures, no snapshots of your summer vacation, no television, and no movies. It’s hard to imagine, but that’s what the world was like until the mid-1800s. That’s when the first cameras were made.

HOW DO CAMERAS WORK?
A basic camera works a lot like your eyes. Try this: First, close your eyes. Now quickly open and shut them. What did you see? You saw an image, or “picture,” from your surroundings. Acamera does the same thing, but it has a shutter instead of eyelids. When you take a picture, the shutter quickly opens and shuts. While the shutter is open, the camera “sees” an image, much like your eyes. The camera captures this picture. A film camera catches the picture using chemicals on film. A digital camera captures the image electronically and stores it in memory or on a computer disk. The first popular photographs, called daguerreotypes, were captured on copper plates in the 1840s. Later, pictures were recorded on glass plates. Flexible film, much like we still use today, replaced glass plates in the late 1800s. Like your eyes, a camera has a lens. A lens is a piece of glass shaped to focus light so the picture will be clear. Some cameras even have automatic focus, just like healthy eyes. If a camera lens is out of focus, the picture will be blurry.

HOW CAMERAS CHANGED THE WORLD
The camera changed the world. Before the camera was invented, people created pictures by painting or drawing. That took time and could be inaccurate. Around 1840, that all changed. The camera allowed people to keep a visual record of their lives and important events. Suddenly, people could see pictures of faraway places. The camera brought the whole world into people’s homes. Photographs began to influence people’s opinions about the world. Cameras brought big changes to family life as well. Before the camera, only wealthy people could afford to pay painters to make portraits. Suddenly, ordinary people could afford to have snapshots of themselves and their children or grandchildren. Later, the motion-picture camera was invented. Thanks to that, we have television and movies.

CAMERAS EVERYWHERE
Today, many people have cameras. Most people use point-and-shoot cameras. A point-and-shoot camera automatically focuses the lens and controls how quickly the shutter opens and closes. Many banks, stores, and schools use security cameras to watch what people are doing. Cameras on highways show traffic patterns. There are even tiny cameras on some computers and cell phones. Cameras are important tools for scientists. Doctors use tiny cameras to look inside the human body. Cameras on satellites orbit Earth, taking pictures of weather patterns. Cameras bring us pictures from the deepest oceans, the insides of volcanoes, and even of distant galaxies in space! Cameras are just about everywhere.

Brain and Nervous System

2009-04-26T18:29:00.000-07:00

What kind of supercomputer can write stories, do math problems, draw pictures, play games, see through eyes, hear someone talking, talk back, and network with devices that make snacks in the microwave oven? Your brain and nervous system can do all these things. Do you think a computer will ever be as powerful as your brain? You think with your brain. Your brain also sends signals through a network called your nervous system. It tells your legs to walk and run. It tells your hands and arms to put popcorn in the microwave. You don’t even have to think about many of the things your brain does. Your brain tells your heart to beat. It tells your lungs to breathe in and out, even when you are sleeping. Your brain also controls your feelings. Such feelings as joy, sadness, love, anger, and fear all come from your brain.

WHAT IS MY BRAIN MADE OF?
Your brain is made of about 100 billion nerve cells. It looks like a lump of pinkish-gray jelly. The surface of the brain is wrinkled, and deep grooves divide it into sections. A network of blood vessels brings oxygen and food to your brain cells and carries away wastes. Your brain is protected by bone called your skull. Liquid and skinlike tissues also protect your brain. When you were born, your brain weighed about  pounds (about 0.35 kilograms). Your brain keeps on growing while you grow up. By the time you reach the age of 20, your brain will weigh about 3
pounds (1.3 kilograms). Your brain has three main parts. The parts are called the cerebrum, the cerebellum, and the brain stem. The cerebrum makes up the largest part of the brain. The cerebellum is underneath the back part of the cerebrum. The brain stem connects with the spinal cord at the bottom of the brain. Your cerebrum and cerebellum are divided into two parts. These parts are called the right brain and the left brain. The right side of your brain controls the left side of your body. The left side of your brain controls the right side of your body. Nerves from the right and left side of your body cross over when they enter your brain.

WHAT DOES THE CEREBRUM DO?
Your cerebrum makes up most of your brain. Your cerebrum solves problems and makes wishes. All of your thinking goes on in your cerebrum. Speech, language, and emotions come from your cerebrum, especially your cerebral cortex. The cerebral cortex is the outer part of the cerebrum. Your cerebrum also gets signals from your senses. Nerves carry the signals. Nerves from your eyes and ears go to parts of the cerebrum that let you see and hear. Nerves carry signals to your cerebrum that let you feel, smell, and taste. Your cerebrum sends messages out along nerves. The messages tell your legs to walk or run. They tell your arm and hand to wave when you see a friend across the street.

WHAT DOES THE CEREBELLUM DO?
Your cerebellum coordinates and fine-tunes your body movements. Your cerebrum might tell your hands and arms to hit a baseball. Your cerebellum controls how you swing the bat and make contact with the ball.
Your cerebellum helps your fingers play the piano, guitar, or violin. It helps you keep your balance when you run, jump rope, or walk along a curb.

WHAT DOES THE BRAIN STEM DO?
Your brain stem takes care of all the things that you do but don’t need to think about doing. It keeps your heart pumping blood. It keeps your lungs breathing air. It makes your eyes blink. It pulls your hand back really fast if you touch a hot pot on the stove.

WHAT IS THE NERVOUS SYSTEM?
Your nervous system consists of the brain, spinal cord, and nerves that run throughout your body. The nervous system carries messages to your muscles and organs. These messages tell your body what to do. Your spinal cord is made of bundles of nerves. It starts in your neck and goes down your back. Nerves go out from the spinal cord to other parts of your body. Nerves from the spinal cord extend to the tips of your fingers and toes. Your spine, or backbone, protects your spinal cord.

Bones and Skeleton

2009-04-26T18:21:00.000-07:00

Squeeze your arm. The outside of your arm is soft, but there is a hard part inside. The hard part is a bone. There are bones in your arms and in your legs. Bones go up the middle of your back. They go around your chest. All of your bones together make up your skeleton. Your skeleton holds your body up. It gives your body its shape. Bones do many other important jobs in your body.

WHAT DO BONES DO?
Many bones protect the soft parts inside your body. Skull bones around your head protect your brain. Rib bones make a cage around your chest. Your rib cage protects your lungs and heart. Muscles hook on to bones. Muscles pull on your bones to make them move. Muscles and bones together let you stand, sit, and walk around.
Blood is made in the center of bones. The center of a bone is filled with bone marrow. Bone marrow is soft. Red and white blood cells are made by bone marrow. Red blood cells carry oxygen to all parts of your body. White blood cells help your body fight germs. Three tiny bones help you hear. The three bones are deep inside your ears. One of these bones is called the stirrup bone. It is the smallest bone in your body.

WHAT ARE BONES MADE OF?
There are two kinds of bone. One kind is called compact bone and the other is called spongy bone. Compact bone is the hard and smooth part on the outside of a bone. The long bones in your arms and legs have lots of compact bone. Spongy bone usually lies under the compact bone. Spongy bone is at the ends of arm and leg bones as well. Bones of the pelvis (hipbone), ribs, breastbone, backbone, and skull also contain spongy bone. Your skeleton also contains cartilage. Cartilage is like bone but softer. It bends easily. There is cartilage in body parts that must be
tough but able to bend. There is cartilage in the tip of your nose and in the outer part of your ear.

WHAT ARE JOINTS?
Joints are the places where two or more bones meet. Most bones are tied together at joints by tough bands called ligaments. Different kinds of joints let you move in different ways. Move your lower arm up and down. Keep your upper arm still. The joint that joins your upper and lower arm is called the elbow. Your elbow works like a hinge. It lets you move your lower arm, but only up and down. Now swing your arm all around from your shoulder. A joint in your shoulder called a ball-and-socket joint lets you move your arm in many directions. Your skull is made of many bones that do not move. They are held together in one solid piece by suture joints.

HOW DO BONES GROW?
Bones grow or change as long as you live. Your head and other parts of your skeleton had a lot of cartilage when you were born. Bones replaced the cartilage as you got older. Bones get thicker and longer as you grow taller. Bones keep growing in teenagers. Bones stop growing longer in adults. Some bones join together as you get older. Your skeleton had more than 300 bones when you were first born. An adult has 206 bones. The longest and strongest bone in adults is the thighbone, in the upper leg.
Bones are replaced a little bit at a time even after they stop growing. This replacement goes on for as long as you live. Your body needs a mineral called calcium to keep strong bones. Milk has lots of calcium. Running and other exercise also helps build strong, thick bones. Some older people have thin, weak bones. Their bones can break easily. Getting enough calcium and exercise can help keep bones from getting weak and thin.

WHAT HAPPENS TO BROKEN BONES?
Sometimes people have accidents that break bones. Maybe they fall out of a tree or down a flight of stairs. Sometimes football players or other athletes break bones when they are playing sports.A doctor has to fix a broken bone. First, an X-ray picture shows the doctor what the broken pieces of bone look like. Then, the doctor fits the broken parts of the bone back together. This is called setting the bone. Sometimes a broken bone must be put back together with wires or pins. A broken bone should not be used until it is healed. The doctor makes a hard case called a cast for an arm or leg with a broken bone. New bone starts to grow around the break. The pieces grow together and heal the broken bone.

Bank

2009-04-26T18:13:00.000-07:00

What would happen if there were no banks? Where would you get money? Would there even be any money? The United States and Canada each have one bank that decides how much money to issue.

WHERE DOES MONEY COME FROM?
The bank in charge of the money supply is called a central bank. Before there was a central bank, lots of banks printed money. But you couldn’t always use your money if you traveled. Stores and banks in other parts of the country might not accept it. And if your bank went out of business, you were out of luck. You lost your savings.
Today, most countries have a central bank that supervises money. In the United States, this bank is the Federal Reserve. In Canada, it’s the Bank of Canada. Because of the Federal Reserve, all Americans use the same dollar bills, quarters, and other money.

WHY DO WE PUT MONEY IN BANKS?
People put money in banks for safety and for convenience. You might keep small amounts of money in a piggy bank. But what would you do with a lot of money? You could hide it. But what if
someone found your hiding place and took your money? What if you forgot where you put it? You’d have other problems, too, especially if you were a grown-up and had to pay bills. You’d have to go to the telephone company and electric company to pay your bills each month. Banks make it easy. You pay your bills from your bank account.

HOW DO BANK ACCOUNTS WORK?
When you put money in a bank, you have a bank account. The bank gives you checks, and then you can pay bills with a check. A check is a piece of paper that is a promise to pay. You sign your name on it. When you pay your telephone bill with a check, the telephone company sends the check to its own bank. The money that you owe the telephone company then goes from your bank account to the telephone company’s bank account. The two accounts can be in different banks in different states.
Banks also issue plastic debit cards. People with accounts at the bank can use these cards to pay for things and get cash from an ATM (automatic teller machine). When your mom or dad shops, they can give the store their debit card. The store’s bank uses information on the bank card to collect the money from your mom’s or dad’s bank account.
A bank account used for writing checks is called a checking account. You can also have a savings account. People usually put money in a savings account when they don’t plan to use the money right way. The bank pays you a small amount for keeping money in a savings account. This money is called interest. Most checking accounts pay no interest.
If a bank is paying 5 percent interest, then you will get 5 cents every year for every dollar in your savings account. That doesn’t seem like a lot of money, but over time it adds up.

WHAT DO BANKS DO WITH MONEY?
Banks lend money to people who need it. Let’s say a woman wants to buy a car. However, she doesn’t have enough money right now to pay for the car. She might go to a bank and ask for a loan. The loan lets her buy the car now and slowly pay the money back to the bank. Before lending her the money, the bank will make sure the woman is able to pay back the loan. It will ask her where she works and how much money she makes. It will charge a small fee, or interest,
on the money it lends.
Today, bank accounts are insured. They weren’t always. Sometimes there were “runs” on banks when a large number of people demanded their money. This action panicked others, who also took their money out of the bank. A run on the bank began, and the bank went broke. People who didn’t get to the bank in time lost their money.
Governments in many countries decided to do something to stop runs on banks. They insured bank accounts so that people would not have to worry about losing their money. If a bank in the United States goes out of business, the insurance repays people the amount in their accounts up to a limit of $100,000 per person.

LOCOMOTIVE

2009-03-12T07:29:00.000-07:00

I INTRODUCTION

Locomotive, any type of self-propelled vehicle used by railroads to pull or push other types of rolling stock, including passenger, freight, and work cars. The locomotive differs from other kinds of self-propelled railroad vehicles in that it is used solely as a power unit and is not designed for carrying passengers or freight, as are trolley cars and some electric cars.

II STEAM LOCOMOTIVES

Steam locomotives can be classified in a number of ways. The most generally used classification, however, is based on the number and arrangement of wheels with which the engine is equipped. Thisclassification gives the number of wheels on the leading bogie, the number of driving wheels, and the number of wheels on the trailing bogie. Thus, a 2-4-0 locomotive is one with a two-wheel leading

bogie, four driving wheels, and no trailing bogie. Many locomotives are also given special type names. Switch engines, used in railroad yards, are usually of the 0-6-0 or 0-8-0 type. Passenger locomotivesinclude the American, 4-4-0; the Northern, 4-8-4; the Atlantic, 4-4-2; the Pacific, 4-6-2; and the Mountain 4-8-2. Freight locomotives include the Mogul, 2-6-0; the Consolidation, 2-8-0; the Decapod, 2-10-0; the Mikado, 2-8-2; and the Santa Fe, 2-10-2. A special type of locomotive used for heavy freight hauling is the articulated, or Mallet, locomotive, which is made up of two or more separate engines joined together, each with its own set of driving wheels. Among the various Mallet types are 0-6-6-0, 0-8-8-0, 2-6-6-2, 2-8-8-2, 2-10-10-2, and 2-8-8-8-2. Until about 1940, steam engines provided the driving power of most locomotives used on U.S. railroads. Subsequently, the steam locomotive became largely obsolete. By the late 1980s, only a few of them, such as those on the narrow-gage tourist routes of Colorado, were operating in the U.S.
III DIESEL-ELECTRIC LOCOMOTIVES
Among important locomotives developed in the 20th century are the electric locomotive, which picks up electrical power from an overhead wire or a third rail laid beside the track, and the diesel-electriclocomotive. In diesel-electric locomotives, known popularly as diesels, diesel engines are used to drive generators or alternators connected to solid-state rectifiers that power electric motors geared to the axles. This type of locomotive eliminates the need for expensive power-transmission lines. Compared to the steam locomotive, it has a greater availability, meaning a higher average of productive hoursper day, because it does not require frequent stops for water, fuel, and other servicing. Other advantages over the steam engine include its relative efficiency in converting fuel into available energy and its ability to develop a much higher proportion of its maximum pulling power at low speeds. Also, while steam locomotives require an engineman and fireman for each locomotive, diesel-electrics can be operated as multiple units by one individual, resulting in longer trains and fewer employees. See also Internal-Combustion Engine.

Diesel-electric locomotives were brought into service in the U.S. in 1925 for yard switching. The first passenger-train diesel unit was put into operation in 1934, and the first unit specifically designed for freight service came into use in 1941. Before World War II, the number of diesel units in service totaled about 800, as against more than 40,000 steam locomotives. In the late 1970s more than 27,000 diesel units were in operation, accounting for almost all U.S. railway motive power. By 1990, advances in technology allowed three new-generation locomotives to do the work of four older ones. Although railroads in 2000 hauled a record 1.5 trillion ton-miles, the locomotive fleet stood at only 20,028. Recent locomotive designs make use of improved turbochargers coupled with more powerful and more fuel-efficient engines. Locomotive control systems have been converted to solid-state electronics, replacing most relay functions. On-board microprocessors control engine speed, fuel injection, and excitation of the alternator. These computers also interconnect with improved systemsto detect slipping or sliding of the driving wheels, producing faster correction and improved adhesion. An additional function of the microprocessor is to monitor performance of all locomotive systems, thereby increasing their reliability and making the correction of problems easier. A recent innovation in the locomotive is the introduction of variable-frequency, variable voltage, three-phase alternating-current traction motors in place of direct-current series motors, reducing unsprung weight and improving wheel adhesion
IV TURBINE-ELECTRIC LOCOMOTIVES
After World War II, extensive research in combustion engineering aided the development of turbine-electric locomotives, in which either gas or steam turbines were used to drive generators powering electric motors. A boiler was required to produce the steam in a steam turbine. In the gas turbine, gas was produced in a combustion chamber directly ahead of the turbine wheel. Either coal or oil may be used as fuel to produce steam or gas for turbine operation. Liquid propane gas has been used experimentally as fuel for gas turbines. All of these types of turbine-powered locomotives have been judged to be uneconomical for general freight rail operations. Only the gas turbine, driving wheelsthrough hydraulic transmission, has continued in service in the 1990s as the motive power in the form of a power car hauling a lightweight passenger train.

CAR SYSTEM

2009-03-12T07:03:00.000-07:00

IINTRODUCTION

Automobile, self-propelled vehicle used primarily on public roads but adaptable to other surfaces. Automobiles changed the world during the 20th century, particularly in the United States and other industrialized nations. From the growth of suburbs to the development of elaborate road and highway systems, the so-called horseless carriage has forever altered the modern landscape. The manufacture, sale, and servicing of automobiles have become key elements of industrial economies. But along with greater mobility and job creation, the automobile has brought noise and air pollution, and automobile accidents rank among the leading causes of death and injury throughout the world. But for better or worse, the 1900s can be called the Age of the Automobile, and cars will no doubt continue to shape our culture and economy well into the 21st century.

Automobiles are classified by size, style, number of doors, and intended use. The typical automobile, also called a car, auto, motorcar, and passenger car, has four wheels and can carry up to six people, including a driver. Larger vehicles designed to carry more passengers are called vans, minivans, omnibuses, or buses. Those used to carry cargo are called pickups or trucks, depending on their size and design. Minivans are van-style vehicles built on a passenger car frame that can usually carry up to eight passengers. Sport-utility vehicles, also known as SUVs, are more rugged than passenger carsand are designed for driving in mud or snow. In 2001 manufacturing plants in more than 35 countries produced 39.5 million passenger cars. About 7.3 million passenger vehicles were produced in North America in 2001. For information on the business of making cars, see Automobile Industry.

The automobile is built around an engine. Various systems supply the engine with fuel, cool it during operation, lubricate its moving parts, and remove exhaust gases it creates. The engine producesmechanical power that is transmitted to the automobile’s wheels through a drivetrain, which includesa transmission, one or more driveshafts, a differential gear, and axles. Suspension systems, which include springs and shock absorbers, cushion the ride and help protect the vehicle from being damaged by bumps, heavy loads, and other stresses. Wheels and tires support the vehicle on the roadway and, when rotated by powered axles, propel the vehicle forward or backward. Steering and braking systems provide control over direction and speed. An electrical system starts and operates the engine, monitors and controls many aspects of the vehicle’s operation, and powers such componentsas headlights and radios. Safety features such as bumpers, air bags, and seat belts help protect occupants in an accident.
II POWER SYSTEM

Gasoline internal-combustion engines power most automobiles, but some engines use diesel fuel, electricity, natural gas, solar energy, or fuels derived from methanol (wood alcohol) and ethanol (grain alcohol). Most gasoline engines work in the following way: Turning the ignition key operates a switch that sends electricity from a battery to a starter motor. The starter motor turns a disk known as a flywheel, which in turn causes the engine’s crankshaft to revolve. The rotating crankshaft causes pistons, which are solid cylinders that fit snugly inside the engine’s hollow cylinders, to move up and down. Fuel-injection systems or, in older cars, a carburetor deliver fuel vapor from the gas tank to the engine cylinders. The pistons compress the vapor inside the cylinders. An electric current flows through a spark plug to ignite the vapor. The fuel mixture explodes, or combusts, creating hot expanding gases that push the pistons down the cylinders and cause the crankshaft to rotate. The crankshaft is now rotating via the up-and-down motion of the pistons, permitting the starter motor to disengage from the flywheel.
A Engine
The basic components of an internal-combustion engine are the engine block, cylinder head, cylinders, pistons, valves, crankshaft, and camshaft. The lower part of the engine, called the engine block, houses the cylinders, pistons, and crankshaft. The components of other engine systems bolt or attach to the engine block. The block is manufactured with internal passageways for lubricants and coolant. Engine blocks are made of cast iron or aluminum alloy and formed with a set of round cylinders.
The upper part of the engine is the cylinder head. Bolted to the top of the block, it seals the tops of the cylinders. Pistons compress air and fuel against the cylinder head prior to ignition. The top of the piston forms the floor of the combustion chamber. A rod connects the bottom of the piston to the crankshaft. Lubricated bearings enable both ends of the connecting rod to pivot, transferring the piston’s vertical motion into the crankshaft’s rotational force, or torque. The pistons’ motion rotatesthe crankshaft at speeds ranging from about 600 to thousands of revolutions per minute (rpm), depending on how much fuel is delivered to the cylinders.

B Engine Types
The blocks in most internal-combustion engines are in-line designs or V designs. In-line designs are arranged so that the cylinders stand upright in a single line over the crankshaft. In a V design, two rows of cylinders are set at an angle to form a V. At the bottom of the V is the crankshaft. In-line configurations of six or eight cylinders require long engine compartments found more often in trucksthan in cars. The V design allows the same number of cylinders to fit into a shorter, although wider, space. Another engine design that fits into shorter, shallower spaces is a horizontally opposed, or flat, arrangement in which the crankshaft lies between two rows of cylinders.
Engines become more powerful, and use more fuel, as the size and number of cylinders increase. Most modern vehicles in the United States have 4-, 6-, or 8-cylinder engines, but car engines have been designed with 1, 2, 3, 5, 12, and more cylinders.
Diesel engines, common in large trucks or buses, are similar to gasoline internal-combustion engines, but they have a different ignition system. Diesels compress air inside the cylinders with greater force than a gasoline engine does, producing temperatures hot enough to ignite the diesel fuel on contact. Some cars have rotary engines, also known as Wankel engines, which have one or more elliptical chambers in which triangular-shaped rotors, instead of pistons, rotate.

C Fuel Supply

The internal-combustion engine is powered by the burning of a precise mixture of liquefied fuel and air in the cylinders’ combustion chambers. Fuel is stored in a tank until it is needed, then pumped to a carburetor or, in newer cars, to a fuel-injection system.

The carburetor controls the mixture of gas and air that travels to the engine. It mixes fuel with air at the head of a pipe, called the intake manifold, leading to the cylinders. A vacuum created by the downward strokes of pistons draws air through the carburetor and intake manifold. Inside the carburetor, the airflow transforms drops of fuel into a fine mist, or vapor. The intake manifold deliversthe fuel vapor to the cylinders, where it is ignited.
All new cars produced today are equipped with fuel injection systems instead of carburetors. Fuel injectors spray carefully calibrated bursts of fuel mist into cylinders either at or near openings to the combustion chambers. Since the exact quantity of gas needed is injected into the cylinders, fuel injection is more precise, easier to adjust, and more consistent than a carburetor, delivering better efficiency, gas mileage, engine responsiveness, and pollution control. Fuel-injection systems vary widely, but most are operated or managed electronically.
D Exhaust System
The exhaust system carries exhaust gases from the engine’s combustion chamber to the atmosphere and reduces, or muffles, engine noise. Exhaust gases leave the engine in a pipe, traveling through a catalytic converter and a muffler before exiting through the tailpipe.
Chemical reactions inside the catalytic converter change most of the hazardous hydrocarbons and carbon monoxide produced by the engine into water vapor and carbon dioxide. The conventional muffler is an enclosed metal tube packed with sound-deadening material. Most conventional mufflers are round or oval-shaped with an inlet and outlet pipe at either end. Some contain partitions to help reduce engine noise.
Car manufacturers are experimenting with an electronic muffler, which uses sensors to monitor the sound waves of the exhaust noise. The sound wave data are sent to a computer that controls speakersnear the tailpipe. The system generates sound waves 180 degrees out of phase with the engine noise. The sound waves from the electronic muffler collide with the exhaust sound waves and they cancel each other out, leaving only low-level heat to emerge from the tailpipe.
E Cooling and Heating System
Combustion inside an engine produces temperatures high enough to melt cast iron. A cooling system conducts this heat away from the engine’s cylinders and radiates it into the air. In most automobiles, a liquid coolant circulates through the engine. A pump sends the coolant from the engine to a radiator, which transfers heat from the coolant to the air. In early engines, the coolant was water. In most automobiles today, the coolant is a chemical solution called antifreeze that has a higher boiling point and lower freezing point than water, making it effective in temperature extremes. Some engines are air cooled, that is, they are designed so a flow of air can reach metal fins that conduct heat away from the cylinders. A second, smaller radiator is fitted to all modern cars. This unit uses engine heat to warm the interior of the passenger compartment and supply heat to the windshield defroster

TRUCK

2009-03-11T09:08:00.000-07:00

1. INTRODUCTION

Truck, motor vehicle designed primarily for hauling cargo or for special work or service purposes. Trucks are usually larger and heavier than automobiles and differ in basic construction. Most modern automobiles have a unibody construction, in which the body itself provides structural support for the vehicle. Trucks, by comparison, are built around a strong metal frame, called a chassis, that supports the rest of the truck. Trucks usually have larger, more powerful engines and stronger suspensions than automobiles have. Large trucks have additional axles and wheels for carrying heavy loads.

Trucks come in many different varieties and are classified by weight, type, and the job they perform. Light trucks have a vehicle weight of up to 6,300 kg (14,000 lb) and are used for light hauling or towing, as well as for everyday transportation. Medium trucks have a weight of up to 15,000 kg (33,000 lb) and are typically used as dump trucks, garbage trucks, local freight-delivery trucks, and utility vehicles. Light and medium trucks are usually powered by gasoline internal-combustion engines, although some may be powered by diesel engines. Heavy trucks are over 15,000 kg (33,000 lb) in weight and are used primarily to pull trailers. Such trucks are often called semis (short for semitrailer), or tractor-trailer trucks, and use large diesel engines for power.

Heavy trucks also includelarge vehicles designed for off-road work, such as large construction and mining trucks. Trucks are also classified by their construction. Straight trucks have the body, or cab, and cargo compartment mounted on the same frame. All light trucks and most medium trucks are straight trucks. In a tractor-trailer truck, the engine and cab are part of the tractor unit. The trailer is a detachable unit, is separate from the tractor, and has its own suspension and wheels. It is joined to the tractor by a hinged platelike mounting device called a fifth wheel.

2. LIGHT TRUCKS

Light trucks include pickup trucks, minivans and full-size vans, and sport utility vehicles (SUVs). The engines, transmissions, and brakes of light trucks are usually similar to those used in passenger cars. Some light trucks offer four-wheel drive for off-road driving or driving on mud or snow. Light trucks are often used as passenger cars are for everyday driving, particularly in rural areas, but they also provide the ability to carry extra cargo when needed. They are used primarily for light hauling and construction work.

Most pickup trucks have a cab with two doors and a single bench seat that holds up to three passengers. Some have extended cabs with a rear seat or additional doors. The cargo area behind the cab is called the bed. It has raised sides and a fold-down tailgate at the back for loading and unloading cargo.

Vans are mechanically similar to pickup trucks, except that the cargo area is enclosed and is part of the body. Vans typically have side doors and a rear door in addition to the front doors. A cargo van is equipped with only a front seat, and the unfinished rear area is reserved for hauling items. Cargo vans.are often used to deliver small parcels and goods in urban areas. A passenger van has multiple seats for carrying extra passengers, as well as side windows. Some have lengthened bodies to carry up to 15 or more passengers and are used like small buses. A conversion van is a cargo van that has been specially modified to carry passengers in luxury. Conversion vans typically have plush interiors, numerous accessories, and individual seats. Minivans are smaller vans intended primarily for hauling six to eight passengers. Minivans have mostly replaced station wagons as a family vehicle. Some minivans have rear-wheel drive, but most have front-wheel drive. They also have the same engines, transmissions, and suspension components as front-wheel-drive passenger cars. In addition to the two front doors, most minivans have one or two sliding side doors and a rear liftgate that opens vertically.

Many light trucks are adapted for special uses. Tow trucks have a derrick and winch for towing vehicles, or a tilting bed for carrying disabled vehicles. Many ambulances are also built on light-truck frames. Large police trucks, nicknamed paddy wagons, have an enclosed confinement area in the back for transporting groups of people who have been arrested. Some police departments in large cities also have specially armored trucks for use by riot teams or bomb disposal units.

3. MEDIUM TRUCKS

Dump Truck

Dump trucks have large open beds for hauling loose material such as gravel

or soil. To empty the bed's contents, a hydraulic lift inside the truck tilts the

bed, dumping the contents behind the truck. Dump trucks are common at

busy construction sites, where large amounts of building materials are

frequently moved.

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Medium-duty trucks have larger engines and frames than do light trucks and often have dual rear wheels or a second rear axle for carrying additional weight. Medium trucks are used primarily for local delivery work, for construction, and by municipal and utility fleets. Dump trucks have a hydraulically powered bed that tilts up and down for hauling and dumping rock, gravel, dirt, and debris. Garbage trucks have a large enclosed compartment and a hydraulic compressor for compacting and hauling refuse. Utility trucks with a boom and basket, nicknamed cherry pickers, are used by utility companies for lifting people up to work on utility lines or trees. Fire engines often have a large internal water tank and pump for fighting fires. Some carry large ladders for reaching the upper floors on multistory buildings. See Fire Fighting.

4. HEAVY TRUCKS

Tractor-trailer Truck

Tractor-trailer trucks, also called semis, haul much of the freight in the United States and a significant percentage of freight in the rest of the world. Trailers can deliver raw materials, refrigerated goods, and finished products to factories, warehouses, and markets. The tractor shown here includes a sleeping compartment so the driver can rest during long trips.

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Heavy trucks have the largest frames and are usually powered by large diesel engines. They have multispeed transmissions, with as many as 18 gears, for pulling heavy loads. Most semis or tractor-trailer trucks have two rear drive axles, called dual tandems, each of which has dual tires on each side. Trucks with this arrangement are nicknamed 18-wheelers because of the total number of wheels under the truck and trailer.

When a heavy truck is assembled, the type of engine, transmission, brakes, axle arrangement, and other equipment used are usually specified by the individual or company that is ordering the truck.

Almost all heavy trucks have power steering, cruise control, and air brakes. Air brakes use compressed air, rather than brake fluid, to activate and release the brakes. An air reservoir in the truck supplies air to the brakes through air hoses. The constant supply of air keeps the brakes released. Pressing on the brake pedal decreases the air pressure and applies the brake. If the brake system malfunctions or if the air hoses become punctured, the brakes apply themselves automatically as a safety precaution. Most heavy trucks are designed for hauling goods over long distances, known as over-the-road hauling, although some are also used for local deliveries.

The truck cab may be positioned behind the engine and hood or located directly over the engine. The latter is called a cab-over design, and it reduces the overall length of the truck-trailer combination. Cab-over trucks are hinged to tilt forward when engine access is necessary for maintenance or repairs. Most long-haul heavy trucks have a sleeper compartment, behind the cab, that contains a bed for the driver. The sleeper compartment is the truck driver’s home away from home and may be equipped with a television, a microwave oven, and a refrigerator. Most heavy trucks are also equipped with citizens band radio for communication. Some truck fleets use satellite tracking equipment to monitor their trucks, so the fleet operator or dispatcher can know a truck’s location at any time.

5. TRAILERS

The large trailers pulled by semi tractors have their own rear suspension and wheels, with the front of the trailer being supported by the fifth wheel on the tractor. Semi trailers also have folding supports under the front that are lowered when the trailer is detached from the tractor and parked. The brakes on the trailer’s axles have air hoses that attach to the tractor’s brake system, so the tractor and trailer brakes work together. Trailers have their own signal, tail, and brake lights, all of which are powered by the tractor’s electrical system.

Trailers come in many different designs, depending on the intended cargo. Enclosed, or standard box-type trailers are used to haul a wide variety of goods and merchandise. Double trailers are often used on roads that have sharp turns. Double trailers resemble two smaller trailers linked together and can maneuver through tight turns more easily than standard trailers can. Size and weight restrictions apply and vary from state to state. In the United States, tractor and single trailer combinations generally must be less than 16 m (53 ft) in length and are limited to a maximum weight of 36,000 kg (80,000 lb). Separate weight limits apply to trailers with single or tandem axles and to double trailers. Maximum trailer height and width are dictated by state law and vary from state to state. For most states, the maximum height is 4.11 m (13.5 ft), and the maximum width is 2.6 m (8.5 ft). In some states, trailers may be equipped with additional wheels and axles to carry heavier loads.

A special type of enclosed trailer is an insulated and refrigerated “refer” unit, used for transporting perishable food items. Refrigerated trailers have a small engine mounted on the trailer for powering the refrigeration system. This allows the refrigeration unit to run continuously, even when the trailer is parked or disconnected from the tractor.

Piggyback trailers are enclosed trailers designed to be mounted on railroad flatcars for cross-country transport. Some have their own wheels and suspension, while others are sealed containers that are lifted off and placed on a trailer chassis. Sealed containers are also used on special ships, called container ships, to transport goods overseas.

Flatbed trailers are used to transport large objects such as construction equipment, industrial machinery, and oversized objects. Such trucks may be equipped with an Oversize Load warning sign and flashing lights, and may be accompanied by an escort vehicle to warn other motorists. Platform trailers are essentially large containers with open tops for transporting produce and grain. Special trailers are also designed for hauling livestock, automobiles, and beverages. Tank trailers, known as tankers, are used to haul chemicals, milk, gasoline, and other liquids. Tankers, as well as other trucks that carry flammable or toxic products, must display special warning emblems to warn police and firefighters in case of an accident.

6. TRUCKING OPERATIONS AND REGULATIONS

Trucking has become the predominant means of delivering all types of goods, accounting for four-fifths of all domestic freight value in the United States. Trucks in the late 1990s hauled 1.5 trillion ton-kilometers of freight annually (a ton-kilometer is the movement of one metric ton over the distance of one kilometer). All types of manufacturing are also dependent on trucking for deliveries of parts and for shipping finished goods.

A trucking trade association estimates that about 9.7 million people in the United States are employed in trucking industry jobs, a figure that includes about 3.1 million professional truck drivers. In 2002 there were about 588,000 trucking companies in the United States. They generated over $580 billion in gross revenues. At the beginning of the 21st century there were about 87.1 million trucks of all types on U.S. highways, out of a total of 217.6 million vehicles. The majority of trucks on the road were light trucks such as pickups and sport utility vehicles (SUVs); only about 21.3 million were trailers and semitrailers. In Canada, about 450,000 trucks are used to carry commercial freight.

Trucks that operate between states must be licensed in each state through which they travel, and their owners must pay road fees in each state. Trucks may be licensed to over-the-road commercial carriers, private delivery companies that operate their own trucks, or private owner-operators. Trailers are licensed separately from tractors and may be owned separately by a different company or fleet. There are about 4.7 million commercial semitrailers in the United States.

Drivers of over-the-road trucks must have a commercial driver’s license, which is obtained by special training and the passing of written and driving examinations. Drivers must also keep written logbooks of their hours and miles traveled, as these are regulated for safety purposes to minimize driver fatigue. Drivers who work for trucking companies are generally paid by the mile, while owner-operators and commercial carriers charge for freight by weight and distance. A typical commercial over-the-road truck is driven over 161,000 km (100,000 mi) a year. Trucking operations are regulated by state and local agencies to ensure safety on the road. Trucks traveling on interstate highways and primary roads must be inspected at weigh stations to make sure they are not overloaded. Drivers are fined if the weight exceeds the limit allowed for each axle. Trucks are also subject to safety inspections and may be put out of service if the brakes, tires, or other safety equipment do not meet standards. Because big trucks are so large and heavy, they cannot stop or maneuver as quickly as cars and light trucks can. Of the almost 42,400 traffic fatalities that occurred in the United States in 2000, about 12 percent involved heavy trucks. In most car-truck accidents, the truck driver often is uninjured or suffers only minor injuries because of the size difference between the vehicles.

7. HISTORY

Various steam-powered carriages and vehicles were built in Europe from 1769 through the late 1800s. However, one of the first gasoline-powered vehicles for hauling cargo was built in 1896 by Gottlieb Daimler in Germany. The earliest trucks were essentially self-powered wagons, most of which had an open driver compartment in the front. In 1898 the Winton Company of Cleveland, Ohio, became one of the first manufacturers of trucks in the United States. In 1903 the first truck show in the United States was held in New York City. In 1911 the first transcontinental coast-to-coast trip by a truck was completed in 66 days.

In the early 1900s trucks were used primarily for local deliveries and limited intercity commerce. Roads were poor, and railroads controlled the long-distance shipping of freight. As roads improved and more highways were built, however, the role of trucking in commerce grew in importance. The Federal-Aid Road Act of 1916 promoted the building of paved roads between cities to facilitate travel and commerce. By the end of World War I (1914-1918), more than 600,000 trucks were in use in the United States. Trucks proved to be an invaluable method for moving both soldiers and supplies during the war, and were also used extensively as ambulances for transporting wounded soldiers.

In 1935 the Congress of the United States passed the Motor Carrier Act to expand the role of the Interstate Commerce Commission (ICC), a federal agency that regulated commerce between the states. The ICC was originally created to regulate commercial rail and water transportation. Its new authority allowed the ICC to establish regulations for trucking companies involved in interstate business. As the nation’s highway system expanded, so did the use of trucking to move goods and produce. The construction of the interstate highway system in the late 1950s and 1960s made long-haul trucking not only practical but also highly competitive with rail freight. In 1980 the trucking industry was deregulated, allowing the establishment of many small independent trucking companies. Deregulation stimulated competition in the trucking industry and generally lowered the cost of shipping freight by

truck.

TANK

2009-03-11T08:17:00.000-07:00

1. INTRODUCTION
Tank, heavily armored military vehicle with cross-country mobility and road speeds up to 97 km/h (60 mph). Tanks travel on tracks or treads, rather than wheels. Tanks are classified as light, medium, and heavy. They range in weight from approximately 14 to 69 metric tons, have at least 15 cm (6 in) of armor plate, and mount cannons ranging from 75 mm to 122 mm in the tank's turret.
The turret is a structure on top of the tank that can rotate 360 degrees, enabling the tank to fire in any direction. In addition, tanks often have both light and heavy machine guns. Light tanks are used for reconnaissance; heavier tanks are used primarily to penetrate or flank enemy defenses. The concept of armor protection dates from antiquity. By the 5th century BC Greek warriors, and sometimes their horses, wore armor. Florentine artist and scientist Leonardo da Vinci designed a crank-operated covered chariot in 1482, but development of an effective, track-laying armored vehicle was only possible after the invention of the internal-combustion engine.

2. THE FIRST TANKS
During World War I (1914-1918) the British developed and used the first armored track-laying vehicles in an attempt to break the deadlock of trench warfare on the Western Front. To maintain secrecy, the vehicles were shipped to the battle zone in crates marked “tanks,” hence the origin of the name.
The tank had no single inventor. The Royal Naval Air Service, which operated some armored cars, originated ideas for “landships.” Simultaneously, British colonel Ernest Swinton proposed the military use of track-laying tractors. Both suggestions came to the attention of the First Lord of the Admiralty, Sir Winston Churchill. He set up a Landships Committee that laid down specifications. By the end of 1915 British engineers had built a prototype landship, nicknamed “mother.” The first battle in which these tanks were employed was the Battle of the Somme, on September 15, 1916, when the British used 49 tanks with disappointing results. Although the military impact was limited, the psychological and propaganda impact was huge.
As the war progressed, armored protection and engines were improved. In November 1917, at the Battle of Cambrai, about 400 British tanks achieved a notable success in breaking through the German lines, helping capture 8,000 of the enemy and 100 guns. In 1918 tanks formed an important element of many attacks during the Allied advances that preceded the armistice ending World War I. The French army had introduced its own tanks in 1917, but the Germans had produced only a handful before the end of the war. The slowness and unreliability of early tanks prevented them from being a decisive weapon in their own right. With the exception of the French Renault light tank, early tanks deployed their armament in their hulls rather than in a rotating turret. However, they achieved enough success to secure their place on the battlefield, and development continued following World War I.

3. TANKS DURING WORLD WAR II
Tanks played an even greater role in warfare during World War II (1939-1945). Early in the warGermany organized tanks, infantry, artillery, and support troops into fast, mobile attack units; theseunits were responsible for many of Germany's early victories in the war. A group of German officers,most notably General Heinz Guderian, had formulated a method of attack that became known asblitzkrieg (lightning war), using mobile forces spearheaded by tanks and supported by dive-bombers.As a result the nations opposing Germany in the war quickly incorporated tanks into their militaryforces. By the middle of the war, tanks were a central part of most infantry units, and played a prominent role in battles in the deserts of northern Africa, in the Union of Soviet Socialist Republics(USSR), and in Europe.Their expert handling of massed tanks enabled the Germans to defeat the Allied forces in 1940. Evenwhen equipped with effective tanks of their own, the Allies tended to employ them clumsily, in small
formations. As the war progressed, the Allies also created large armored formations challenging the Germans on their own terms. The Soviet Union was particularly successful in this. From 1943 through 1945 the Soviets launched a series of successful tank-led offensives, in particular at the Battle ofKursk, culminating in the final defeat of Germany in Berlin. The tank production of the Allies massivelyoutstripped that of Germany, also contributing to Germany’s defeat. World War II witnessed huge advances in tank technology. In 1939 most armies had employed considerable numbers of small light tanks, largely because they were relatively cheap to build.
However, events proved that their thin armor-plating made them a liability. A race ensued to produce tanks with an optimum combination of firepower, speed, and armored protection. The Soviet T-34tank, the M4 tank of United States forces, and the German Panzer tank were notably successful inbalancing these requirements. However, the heavier tanks developed during World War II, such as the Soviet’s Joseph Stalin series,or the British Centurion (which appeared just too late to see action), foreshadowed modern tank development. Powerful though these vehicles were, the war also proved that they required the support of other arms. Tanks attempting to fight alone were vulnerable to antitank guns and to a range of newly developed infantry antitank weapons. The most effective of these antitank weapons employedmunitions that could penetrate very thick armor
4. LATER TANK DEVELOPMENT

M1 Abrams Tank

Tanks, chiefly used to combat ground targets, airplanes, and other armored vehicles, can weigh as much as 25 metric tons and travel at speeds of up to 80 km/hr (50 mph) on level ground. The M1 Abrams tank, shown here, was used in the 1991 Persian Gulf War.

In the era following World War II, the world’s major armies all maintained large fleets of tanks. During the Cold War massive armored forces faced each other along the borders dividing Western Europe and Eastern Europe. However, the only major use of tanks after 1945 occurred in the Middle East, during the Six-Day War of 1967, the Arab-Israeli War of 1973, the Persian Gulf War of 1991, and the UnitedStates-led invasion of Iraq in 2003.
Modern tanks are beginning to differ significantly from tanks used toward the end of World War II, dueprimarily to computer technology. Modern tanks are better armored, faster, and use more powerful cannons. They also can maneuver and fight at night or in thick smoke using night-vision devices. Tank commanders can accurately determine their position with computerized maps and the global positioning system. In addition, computerized firing controls, combined with stabilized gun mounts, enable modern tanks, such as the M1 Abrams tank of the U.S. military, to accurately fire on targets even while moving.

d heavy. They range in weight from approximately 14 to 69 metric tons, have at least 15 cm (6 in) of armor plate, and mount cannons ranging from 75 mm to 122 mm in the tank's turret.
The turret is a structure on top of the tank that can rotate 360 degrees, enabling the tank to fire in any direction. In addition, tanks often have both light and heavy machine guns. Light tanks are used for reconnaissance; heavier tanks are used primarily to penetrate or flank enemy defenses. The concept of armor protection dates from antiquity. By the 5th century BC Greek warriors, and sometimes their horses, wore armor. Florentine artist and scientist Leonardo da Vinci designed a crank-operated covered chariot in 1482, but development of an effective, track-laying armored vehicle was only possible after the invention of the internal-combustion engine.

2. THE FIRST TANKS
During World War I (1914-1918) the British developed and used the first armored track-laying vehicles in an attempt to break the deadlock of trench warfare on the Western Front. To maintain secrecy, the vehicles were shipped to the battle zone in crates marked “tanks,” hence the origin of the name.
The tank had no single inventor. The Royal Naval Air Service, which operated some armored cars, originated ideas for “landships.” Simultaneously, British colonel Ernest Swinton proposed the military use of track-laying tractors. Both suggestions came to the attention of the First Lord of the Admiralty, Sir Winston Churchill. He set up a Landships Committee that laid down specifications. By the end of 1915 British engineers had built a prototype landship, nicknamed “mother.” The first battle in which these tanks were employed was the Battle of the Somme, on September 15, 1916, when the British used 49 tanks with disappointing results. Although the military impact was limited, the psychological and propaganda impact was huge.
As the war progressed, armored protection and engines were improved. In November 1917, at the Battle of Cambrai, about 400 British tanks achieved a notable success in breaking through the German lines, helping capture 8,000 of the enemy and 100 guns. In 1918 tanks formed an important element of many attacks during the Allied advances that preceded the armistice ending World War I. The French army had introduced its own tanks in 1917, but the Germans had produced only a handful before the end of the war. The slowness and unreliability of early tanks prevented them from being a decisive weapon in their own right. With the exception of the French Renault light tank, early tanks deployed their armament in their hulls rather than in a rotating turret. However, they achieved enough success to secure their place on the battlefield, and development continued following World War I.

3. TANKS DURING WORLD WAR II
Tanks played an even greater role in warfare during World War II (1939-1945). Early in the warGermany organized tanks, infantry, artillery, and support troops into fast, mobile attack units; theseunits were responsible for many of Germany's early victories in the war. A group of German officers,most notably General Heinz Guderian, had formulated a method of attack that became known asblitzkrieg (lightning war), using mobile forces spearheaded by tanks and supported by dive-bombers.As a result the nations opposing Germany in the war quickly incorporated tanks into their militaryforces. By the middle of the war, tanks were a central part of most infantry units, and played a prominent role in battles in the deserts of northern Africa, in the Union of Soviet Socialist Republics(USSR), and in Europe.Their expert handling of massed tanks enabled the Germans to defeat the Allied forces in 1940. Evenwhen equipped with effective tanks of their own, the Allies tended to employ them clumsily, in small
formations. As the war progressed, the Allies also created large armored formations challenging the Germans on their own terms. The Soviet Union was particularly successful in this. From 1943 through 1945 the Soviets launched a series of successful tank-led offensives, in particular at the Battle ofKursk, culminating in the final defeat of Germany in Berlin. The tank production of the Allies massivelyoutstripped that of Germany, also contributing to Germany’s defeat. World War II witnessed huge advances in tank technology. In 1939 most armies had employed considerable numbers of small light tanks, largely because they were relatively cheap to build.
However, events proved that their thin armor-plating made them a liability. A race ensued to produce tanks with an optimum combination of firepower, speed, and armored protection. The Soviet T-34tank, the M4 tank of United States forces, and the German Panzer tank were notably successful inbalancing these requirements. However, the heavier tanks developed during World War II, such as the Soviet’s Joseph Stalin series,or the British Centurion (which appeared just too late to see action), foreshadowed modern tank development. Powerful though these vehicles were, the war also proved that they required the support of other arms. Tanks attempting to fight alone were vulnerable to antitank guns and to a range of newly developed infantry antitank weapons. The most effective of these antitank weapons employedmunitions that could penetrate very thick armor
4. LATER TANK DEVELOPMENT

M1 Abrams Tank

Tanks, chiefly used to combat ground targets, airplanes, and other armored vehicles, can weigh as much as 25 metric tons and travel at speeds of up to 80 km/hr (50 mph) on level ground. The M1 Abrams tank, shown here, was used in the 1991 Persian Gulf War.
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In the era following World War II, the world’s major armies all maintained large fleets of tanks. During the Cold War massive armored forces faced each other along the borders dividing Western Europe and Eastern Europe. However, the only major use of tanks after 1945 occurred in the Middle East, during the Six-Day War of 1967, the Arab-Israeli War of 1973, the Persian Gulf War of 1991, and the UnitedStates-led invasion of Iraq in 2003.
Modern tanks are beginning to differ significantly from tanks used toward the end of World War II, dueprimarily to computer technology. Modern tanks are better armored, faster, and use more powerful cannons. They also can maneuver and fight at night or in thick smoke using night-vision devices. Tank commanders can accurately determine their position with computerized maps and the global positioning system. In addition, computerized firing controls, combined with stabilized gun mounts, enable modern tanks, such as the M1 Abrams tank of the U.S. military, to accurately fire on targets even while moving.