To understand the role of living organisms in the water cycle, you must first understand the general processes of the water cycle and its non-living components. The three main steps of the cycle are condensation, precipitation and evaporation. Condensation is the change from a gas to a liquid; this occurs in the atmosphere under conditions of low temperature and pressure, resulting in the formation of clouds. Precipitation occurs when the water droplets formed among the clouds become large and heavy and fall to the ground, most commonly in the form of rain. Other forms of precipitation include snow, sleet and hail, and depend upon other atmospheric conditions. The final step of the cycle is evaporation, during which water from the Earth's surface evaporates into gas, which rises back into the atmosphere. The greatest sources of evaporation are large bodies of water, such as lakes and the ocean. Living organisms, mainly plants and animals, also contribute to this last step of the cycle through processes, such as respiration, perspiration and transpiration.