Water is the most common substance on Earth. It covers more than 70 percent of the Earth's surface. It fills the oceans, rivers, and lakes, and is in the ground and in the air we breathe. Water is everywhere. Without water, there can be no life. In fact, every living thing consists mostly of water. Your body is about two-thirds water. A chicken is about three-fourths water, and a pineapple is about four-fifths water. Most scientists believe that life itself began in water-in the salty water of the sea. Ever since the world began, water has been shaping Earth. Rain hammers at the land and washes soil into rivers. The oceans pound against the shores, chiseling cliffs and carrying away land. Rivers knife through rock, carve canyons, and build up land where they empty into the sea. Glaciers plow valleys and cut down mountains. Water helps keep Earth's climate from getting too hot or too cold. Land absorbs and releases heat from the sun quickly. But the oceans absorb and release the sun's heat slowly. So breezes from the oceans bring warmth to the land in winter and coolness in summer.

Interesting facts about water Water in our daily lives Nature's water cycle The water supply problem City water systems Fresh water from the sea What water is and how it behaves Water and the course of history

Throughout history, water has been people's slave-and their master. Great civilizations have risen where water supplies were plentiful. They have fallen when these supplies failed. People have killed one another for a muddy water hole. They have worshiped rain gods and prayed for rain. Often, when rains have failed to come, crops have withered and starvation has spread across a land. Sometimes the rains have fallen too heavily and too suddenly. Then rivers have overflowed their banks, drowning large numbers of people and causing enormous destruction of property.

Today, more than ever, water is both slave and master to people. We use water in our homes for cleaning, cooking, bathing, and carrying away wastes. We use water to irrigate dry farmlands so we can grow more food. Our factories use more water than any other material. We use the water in rushing rivers and thundering waterfalls to produce electricity.

Our demand for water is constantly increasing. Every year, there are more people in the world. Factories turn out more and more products, and need more and more water. We live in a world of water. But almost all of it-about 97 percent-is in the oceans. This water is too salty to be used for drinking, farming, and manufacturing. Only about 3 percent of the world's water is fresh (unsalty). Most of this water is not easily available to people because it is locked in ice that covers Antarctica, Greenland, and the waters of the north polar region. But there is still enough to meet people's needs. There is as much water on Earth today as there was when dinosaurs inhabited the planet millions of years ago.

There is as much water on Earth today as there ever was-or ever will be. Almost every drop of water we use finds its way to the oceans. There, it is evaporated by the sun. It then falls back to Earth as rain. Water is used and reused over and over again. It is never used up.

Although the world as a whole has plenty of fresh water, some regions have a water shortage. Rain does not fall evenly over Earth. Some regions are always too dry, and others too wet. A region that usually gets enough rain may suddenly have a serious dry spell, and another region may be flooded with too much rain.

Some regions have a water shortage because the people have managed their supply poorly. People settle where water is plentiful-near lakes and rivers. Cities grow, and factories spring up. The cities and factories dump their wastes into the lakes and rivers, polluting them. Then the people look for new sources of water. Shortages also occur because some cities do not make full use of their supply. They have plenty of water but not enough storage tanks, treatment plants, and distribution pipes to meet the people's needs. As our demand for water grows and grows, we will have to make better and better use of our supply. The more we learn about water, the better we will be able to meet this challenge.

Water in our daily lives

Every plant, animal, and human being needs water to stay alive. This is because all the life processes-from taking in food to getting rid of wastes-require water. But people depend on water for more than just to stay alive. We also need it for our way of life. We need water in our homes-to brush our teeth, cook food, and wash dishes. We need water in our factories-to manufacture almost everything from automobiles to zippers. We need water for irrigation-to raise crops in regions that do not get enough rain.

Water in living things. Every organism (living thing) consists mostly of water. The human body is usually made up of 50 to 75 percent water. A mouse is about 65 percent water. An elephant and an ear of corn are about 70 percent water. A potato and an earthworm are about 80 percent water. A tomato is about 95 percent water.

All living things need a lot of water to carry out their life processes. Plants, animals, and human beings must take in nutrients (food substances). Watery solutions help dissolve nutrients and carry them to all parts of an organism. Through chemical reactions, the organism turns nutrients into energy, or into materials it needs to grow or to repair itself. These chemical reactions can take place only in a watery solution. Finally, the organism needs water to carry away waste products.

Every living thing must keep its water supply near normal, or it will die. Human beings can live without food for several weeks, but they can live without water for only about one week. If the body loses more than 20 percent of its normal water content, a person will die painfully. Human beings should take in about 2 1/2 quarts (2.4 liters) of water a day. This intake can be in the form of beverages we drink, or water in food.

Water in our homes. In our homes, we use far more water than the amount we need simply to stay alive. We require water for cleaning, cooking, bathing, and carrying away wastes. For many people, such water is a luxury. Millions of homes in Asia, Africa, and South America have no running water. The people must haul water up by hand from the village well, or carry it in jars from pools and rivers far from their homes.

The United States has more homes with kitchen faucets and flush toilets than any other country. On the average, every American uses more than 100 gallons (380 liters) of water a day in the home. It takes up to about 4 gallons (15 liters) of water to flush a toilet. It takes 20 to 30 gallons (76 to 114 liters) to take a bath, and each minute under a shower takes at least 5 gallons (19 liters). It takes more than 15 gallons (57 liters) of water to wash a day's dishes, and about 40 gallons (152 liters) to run an automatic washing machine.

Water for irrigation. Most of the plants that people raise need great quantities of water. For example, it takes 115 gallons (435 liters) of water to grow enough wheat to bake a loaf of bread. People raise most of their crops in areas that have plenty of rain. But to raise enough food for their needs, people must also irrigate dry areas. The rainfall that crops use to grow is not considered a water use, because the water does not come from a country's supply. Irrigation, on the other hand, is a water use because the water is drawn from a nation's rivers, lakes, or wells.

The water a nation uses for irrigation is important to its water supply because none of the water remains for reuse. Plants take in water through their roots. They then pass it out through their leaves into the air as a gas called water vapor. Winds carry away the vapor, and the liquid water is gone. On the other hand, nearly all the water used in our homes is returned to the water supply. The water is carried by sewer pipes to treatment plants, which return the water to rivers so it can be used again.

The United States uses about 140 billion gallons (530 billion liters) of water a day for irrigation. This is enough water to fill a lake 5 miles (8 kilometers) long, 1 mile (1.6 kilometers) wide, and 130 feet (40 meters) deep. About 40 percent of all the water used in the United States is used for irrigation. For a discussion of irrigation systems,

Water for industry. Industry uses water in many ways. It uses water for cleaning fruits and vegetables before canning and freezing them. It uses water as a raw material in soft drinks, canned foods, and many other products. It uses water to air-condition and clean factories. But most of the water used by industry is for cooling. For example, water cools the steam used in producing electric power from fuel. It cools the hot gases produced in refining oil, and the hot steel made by steel mills.

About 38,000 gallons (144,000 liters) of water are required to make a ton of steel or a ton of paper. Manufacturers use about 7 gallons (27 liters) of water to refine 1 gallon (3.8 liters) of gasoline. It takes about 15 gallons (57 liters) of water to brew 1 gallon of beer.

In the United States, factories and steam-producing power plants draw about 160 billion gallons (600 billion liters) of water every day from wells, rivers, or lakes. This total accounts for 48 percent of all the water used in the country. Power plants use 80 percent of this water. In addition, many factories buy water from city water systems.

Although industry uses a lot of water, only 6 percent of it is consumed. Most of the water used for cooling is piped back to the rivers or lakes from which it is taken. The water consumed by industry is the water added to soft drinks and other products, and the small amount of water that turns to vapor in the cooling processes.

Water for power. People also use water to produce electric power to light homes and to run factories. Electric power stations burn coal or other fuel to turn water into steam. The steam supplies the energy to run machines that produce electricity. Hydroelectric power stations use the energy of falling water from waterfalls and dams to produce electricity.

Water for transportation and recreation. After people learned to build crude small boats, they began using rivers and lakes to carry themselves and their goods. Later, they built larger boats and sailed the ocean in search of new lands and new trade routes. Today, people still depend on water transportation to carry such heavy and bulky products as machinery, coal, grain, and oil.