In nature, water continuously moves between Earth’s surface and the air in a process called the water cycle. Most of Earth’s water is contained in oceans. Energy from the Sun evaporates water, making it rise. Some water droplets and ice crystals form clouds. When these grow large and heavy, the water falls to Earth’s surface as precipitation. This can take the form of snow, rain, hail, or sleet. Water can be stored as groundwater or it can run off and flow downhill.