Passage: 'The water cycle describes how water moves between the oceans, the air, and the land. Heat from the sun causes water to evaporate from oceans, lakes, and rivers. As the vapour rises, it cools and condenses into tiny droplets that form clouds. When the droplets grow heavy, they fall as rain, snow, or hail. Some of this water flows over the land into rivers, and some soaks into the ground, eventually returning to the sea.' What does the passage mainly describe?