The USGS Water Science School
The sun is what makes the water cycle work. The sun provides what almost everything on Earth needs to go—heat. Heat causes water to change forms from liquid (water) to solid (ice) and to gas (vapor in the air).
Here is a picture of the Earth with the sun in the background. The sun might look small, but it is actually much larger than the Earth. It only looks small because it is so far away (93 million miles). Even that far away, you can still feel the heat.