Water on the Earth is always moving in a cycle. The sun heats the surface of seas, rivers and lakes, and some of the water turns into a gas called water vapor and rises into the air. This is called evaporation. High in the sky, the air is colder, so the vapor cools and turns back into tiny drops of water. These drops come together to form clouds. When the drops become heavy, they fall as rain, snow or hail. Some of this water runs into rivers and returns to the sea, and some goes into the ground, where plants and wells use it. Then the cycle begins again. Without this cycle, fresh water would not be renewed, and life on land would not be possible.

Question: What is the main idea of the passage?