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!