The earth is a closed system, so the amount of moisture on our planet has not changed. The water you drink today is the same water that the dinosaurs drank millions of years ago. The water cycle is the continuous movement of water on, above, and below the surface of the Earth.
The sun controls the cycle; it heats the water's surface, which evaporates and turns to water vapor. This cools and rises, becoming clouds, and ultimately condenses into water droplets. Then we see the water again in the form of rain, snow, sleet, or hail.
Some precipitation evaporates back into the atmosphere, but when it hits the ground, it becomes runoff, which can freeze into snow caps or glaciers. Or it sinks into the ground and accumulates in large deposits of useful groundwater.
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