The hydrosphere is made up of all the water on Earth. Water exists in three forms-liquid, solid, and water vapor-and changes from one form to another. Water moves around Earth in the water cycle in which some of the water in oceans and rivers turns into water vapor that goes into the air to form clouds. The water is then returned to the surface of Earth as rain or snow. This cycle repeats over and over. Water is necessary for life on Earth to exist. Some people are not careful about keeping our water clean, and others help clean it up. Scientists are working to find new ways to keep Earth's water clean. Pronunciation guide. 24 pages filled with engaging, colorful illustrations. Reading Level 1-3, Interest Level 2-5.