The water cycle is a term for the movement of water between the Earth's surface, the sky and underground. Water evaporates due to heat from the sun; it condenses in clouds and forms rain; the rain forms streams, rivers and other reservoirs which then evaporate again.

The sun is a single star which is at the center of our solar system. The sun provides heat and light energy for all of the planets in the solar system, including planet Earth.

Solar energy takes the form of radiant heat and light emanating from the sun. In the water cycle, the heat and light of solar energy cause water to melt or evaporate, changing the water from a solid or liquid form to a vapor.

Although solar energy is the main energy source for the water cycle, many other kinds of energy are involved as water cycles among solid, liquid and vapor states. Water falling from the sky as rain has kinetic energy (motion-related energy), for example.