The water from the water sources on the surface of the earth evaporates and rise high up in the sky. As they reach height, they start cooling down and convert to tiny droplets of water. These water droplets join together to form heavy and dense clouds. When the clouds become heavy and cannot hold more water, the droplets fall on the earth in different forms such as rain, hail, frost, fog, sleet or snow. This process of falling down of water in different forms from the surface of the earth is called precipitation.