A "closed system" consists of a container that allows energy, but not matter, to pass back and forth across the walls of the container. The Earth's atmosphere, oceans, and land surface act as an almost closed system. Water moves along a variety of pathways in this closed system. This system of movement is called the "water cycle."

There is one main loop in the water cycle. Water evaporates at the ocean surface and then moves as water vapor to the continents. The water vapor condenses and falls as precipitation onto the continents. The water then runs back into the ocean in streams and rivers. Of course, there is rainfall onto the oceans, and there is evaporation of water from the continents. There really is a cycle, however, because there is an excess of evaporation over precipitation on the oceans, and there is an excess of precipitation over evaporation on the continents.