Most living things depend on freshwater for survival. Only three percent of all Earth's water is freshwater. Sixty-seven percent of the world's freshwater is frozen in polar icecaps and glaciers. That makes it unavailable for use by most species. It is constantly changing among these three forms, or phases.
Water moves around Earth as it changes form.
The movement of water occurs in a group of repeating events. The group of repeating events is called the water cycle or hydrologic cycle. It is diagramed in the figure below. Heat from the sun causes water in oceans, lakes, swamps, rivers, plants, and even our own bodies to evaporate into water vapor. Water vapor in the atmosphere forms clouds as it cools and condenses .