The water cycle describes the continuous movement of water on, above, and below the surface of the Earth. Water can change between liquid, solid, and gas states in this cycle. In its purest form, water is a chemical compound made up of two hydrogen atoms and one oxygen atom. Bodies of flowing water include rivers, creeks, brooks, and canals, which vary in size and flow. Bodies of non-flowing water include oceans, lakes, ponds, reservoirs, sloughs, marshes, and estuaries, and also vary in size and salinity.