The water cycle
The water on the planet has been nearly the same for billions of years. What changes is where it is: in the oceans, in clouds, in rivers, in the soil or inside living things. The path between those places is called the water cycle.
The cycle begins with evaporation. Sunlight warms seas, lakes and rivers, and some of the water turns into vapour. As it rises the vapour cools, condenses into tiny droplets and forms clouds; when the droplets grow heavy they fall as rain or snow.
Back on the surface, water runs into rivers and lakes, seeps into the ground or is taken up by plants. Sooner or later it evaporates again, and the cycle starts over with no beginning and no end.