Bodies of water on Earth are maintained by the atmosphere keeping the water cycle of evaporation, condensation and precipitation functioning so that water can be replenished. Also, the gravitational pull of the Earth keeps water from leaving the planet.
On the Earth, water is always moving and changing form. It can be found as liquid or vapor, and is constantly in a state of fluctuation. The atmosphere that envelops our planet keeps water from escaping as well as maintaining the constant motion of the water cycle.
The first step of the water cycle is evaporation. This occurs when the energy among water molecules is transferred due to various causes including heat and pressure. This enables some of the molecules to break free from the liquid form and become vapor. This vapor is then pulled into the atmosphere, where it cools and is returned to a liquid state. It condenses at very high altitudes, where it becomes the clouds in the sky. Clouds release the condensed water molecules in a process called precipitation, otherwise known as rain. The rain replenishes the oceans, lakes, and rivers from which it came. The water cycle is an ongoing event that maintains the bodies of water on this planet.