Water can be used again and again because of something in nature called the water cycle. Basically, the amount of water on the earth stays the same. But because of the water cycle, it is always moving. About 70% of the earth's surface is covered by water. Most of this water is salt water in the oceans. As water in the oceans is heated by the sun, it evaporates. That is, it changes into the form of a gas. This gas, called water vapor, rises and becomes part of the air. When this happens, salt from the ocean water is left behind. As water vapor rises, it becomes cooled and helps form clouds. These clouds are blown over the land. When enough eater vapor collects in the clouds and when they become cool enough, the water vapor turns into rain or snow. Rain and snow help fill the rivers and lakes that supply cities, homes, and industries with water. And water from moist rivers and lakes finds its way back into the oceans. Things that might interfere with the water cycle are: water pollution, damming a river, and reducing the amount of groundwater through overpumping of wells.

Some parts of air go through a natural cycle. Air is made up of a mixture of gases. By volume, about 78% of air is made up of nitrogen. About 21% is oxygen. The remaining 1% is made up of argon, carbon dioxide, helium, hydrogen, and other gases. As far as living things are concerned, the three most important gases in air are nitrogen, oxygen, and carbon dioxide. Nitrogen helps make up the cells of living things. Living things, however, do not get nitrogen directly from air. Certain bacteria that live in soil and in some plants can combine nitrogen from air with other chemicals. They are combined to make compounds known as nitrates. Nitrates provide plants with the nitrogen they need to grow. There is another way which nitrogen from air gets into soil. During thunderstorms, electricity from lightning causes nitrogen to form a compound with oxygen. This compound is then carried to soil by rain. As plants and animals die and decay, they become part of soil. Certain bacteria in soil help cause plants and animals to decay. As they decay, the nitrogen that was part of their body is given off as a gas into the air. Thus, the nitrogen cycle is completed, allowing nitrogen to be used again. Oxygen and carbon dioxide are involved in a combined cycle. Oxygen from the air is needed to help utilize food in the cells of plants and animals. As oxygen is used by the cells, many "waste" products are formed. One of these is carbon dioxide. Carbon dioxide is a compound of carbon and oxygen. Plants, however, also use carbon dioxide from the air in making their food. As carbon dioxide is used by plants, the carbon becomes part of the food. Oxygen is given off and becomes part of the air.