Water

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.