All of the water on the planet is part of the same system. Right now, there is water in rivers, lakes, oceans, swimming pools, your body, under the ground and even in clouds. The same water that is in a swimming pool or at the beach today might be a part of the clouds tomorrow. This is because of the water cycle. The water cycle relies on evaporation and transpiration to turn liquid water into a gas, like oxygen in the air you breathe. When water evaporates, it rises in the sky. The water vapor (water that is a gas instead of liquid) collects to form clouds. The clouds get heavy and eventually the water falls back to the Earth as rain, snow and sleet. This cycle is always happening and it is necessary to keep all living things alive.