Once the water from the water bodies starts to heat up, it evaporates. The water vapor is not as dense and heavy as the air itself, so it goes up into the air, or rather the atmosphere. As it goes up, the water vapor reaches the heights where the temperature is cold enough for it to start to condensate. As the water vapor starts to condensate, small, miniature drops of water form, thus forming the clouds. The clouds than release them because of the gravitational pull, and they get back on the Earth, be it on land, or in the water bodies.
