Most of the water in the universe is created as a byproduct of star formation. The gas and dust that flows out of star nurseries contains clouds of hydrogen (H) and oxygen (O) that react together to form water (H₂O). When the Earth formed by the attraction of many wet dust particles, the water was also trapped. Much of this water eventually made its way to the surface of the Earth. Therefore, our planet has a relatively finite amount of water from its formation. On the surface of the earth, water is constantly being recycled between the ocean, atmosphere, continents, animals and plants as part of a great hydrological cycle.