Earth has a limited water supply that is constantly recycled through the water cycle. Water exists in three states: solid, liquid, and gas. The water cycle involves evaporation as water changes from liquid to gas and collects in the air. It also involves condensation as water vapor in the air condenses to form liquid water clouds. Precipitation occurs when clouds become too heavy and water falls back to Earth as rain, snow, sleet or hail. The water then collects on the ground or in bodies of water, and some seeps underground as groundwater.