Water what? You know, the water cycle you learn in elementary school: how the sun heats up and evaporates water in the ocean and seas, then the water condenses into clouds and then rains back to earth, and so on and so on.
You can probably imagine why the water cycle is important, but have you given a thought as to how big this process is? Like, for example, how much water evaporates from the ocean every year anyhow? Try 92 quadrillion gallons. That's 92,000,000,000,000,000.