The water cycle (also called the hydrologic cycle) is the continuous movement and storage of water across the earth in all forms: liquid, solid (ice) and gas (water vapor). There is no start or end to the water cycle, but for explanation purposes we will start at the sun. Solar radiation from the sun heats the ocean and causes evaporation. This water vapor then enters the atmosphere. As it reaches the upper atmosphere, it cools and condenses forming clouds. Precipitation then falls to the ground and is either stored in ice and snow, runs off into streams, or infiltrates through the ground. All water eventually makes it back to the ocean and starts the cycle again. The different stages in this cycle can take from seconds to thousands of years. Most of the water on earth (97%) is stored in the oceans.
Adding or subtracting heat makes the cycle work. If heat is added to ice, it melts. If heat is added to water, it evaporates. Evaporation turns liquid water into a gas called water vapor. If heat is taken away from water vapor, it condenses. Condensation turns water vapor into a liquid. If heat is taken away from liquid water, it freezes to become ice.
Other, smaller parts of the cycle include plant transpiration. As plants open their pores to take in carbon dioxide for photosynthesis, water vapor is released, cooling the plant. Transpiration can be thought of as plants sweating.