The Earths water is constantly changing form, altering between vapour, liquid, and ice. Because Earths environment is a closed system, the water on the planet today is the same water that existed millions of years ago. During Earths water cycle, surface water is warmed by the sun and evaporates (turns into vapour). This vapour rises up into the atmosphere until it reaches cold air. At this point, depending on the air temperature, it can turn back into either liquid or ice crystals. Then, the Earths gravity plays its part by pulling the water back to Earth where it can fall onto salt or fresh water sources or directly onto the ground.
