The Water Cycle is a process which consists of evaporation, transpiration, condensation, and precipitation. The whole process is controlled by the sun, which produces kinetic energy or heat energy. Evaporation occurs when the kinetic energy of the water molecules increases, causing the molecules to move more quickly, and undergo a liquid-to-gas phase change. Evaporation is stimulated by heat or sunlight. Transpiration is evaporation of water through the leaves of plants. The next part of the cycle is condensation and this is when gas molecules slow down, release energy, and turn into water molecules. Precipitation, is one form in which the condensed water molecules return to the earth (in the form of rain, snow, sleet, hail, fog, or dew). Most precipitation falls in either coastal areas or in high elevations. Some of the water which falls in high elevations becomes runoff water, the water which runs over the ground to lower elevations to form rivers, lakes, and fertile valleys. Sometimes this water collects nutrients from the soil it goes over making valleys fertile for plant growth.