Water molecules enter the atmosphere as water vapor through evaporation from the ocean or other bodies of water or through transpiration (evaporation from leaves of plants. If the air carrying the water vapor cools, it condenses into cloud-forming droplets that may precipitate to rain, snow, sleet or hail. Some precipitation flows along the the surface of the land and is called runoff. Runoff can enter a river or stream and be carried to an ocean or late. Precipitation can also be absorbed into the soil as groundwater. Groundwater can be taken up by plants or flow into streams, rivers, lakes, oceans, or underground reservoirs.