The natural circulation of water on the earth is called the hydrologic cycle. Water cycles from bodies of water, via evaporation to the atmosphere, and eventually returns to the oceans as precipitation, runoff from streams and rivers, and groundwater flow. Water molecules are transformed from liquid to vapor and back to liquid within this cycle. On land, water evaporates from the soil or is taken up by plant roots and eventually transpired into the atmosphere through plant leaves; the sum of evaporation and transpiration is called evapotranspiration.

Water is recycled continuously. The molecules of water in a glass used to quench your thirst today, at some point in time may have dissolved minerals deep in the earth as groundwater flow, fallen as rain in a tropical typhoon, been transpired by a tropical plant, been temporarily stored in a mountain glacier, or quenched the thirst of people...