The water in the rivers,lakes and oceans become steam in the air,and they mix with the dust,then forms the clouds.After more and more clouds get together,they become dark clouds.When it becomes heary enough,it will fall down in the form of rain.The rain water flow together,some of them flow to the rivers and lakes,which finally run into the seas,they are the surface water.Some parts leak into the undergroud and forms spring,these are called undergroud water.All of them will provide the water to human beings.