The earth has a limited amount of water.That water keeps going around and around and around and around,which we call the "Water Cycle".There are three parts in the water cycle----evaporation,condensation and precipitation.In detail,when the sun heats up water in rivers or lakes or the ocean and turns it into vapor or steam.The water vapor or steam leaves the river,lake or ocean and goes into the air.Water vapor in the air gets cold and changes back into liquid,forming clouds.when the cloud is big enough,it rains.We should protect the enviroment to balence the water cycle.Otherwise,there will be a flood when too much precipitation.On the contary,there wil be a draught when too little evaporation.