Water is a continuous cycle, powered by the sun.
Solar energy causes water to evaporate from our seas, lakes and rivers, returning it to the atmosphere as water vapour. Water vapour is also formed through transpiration; this is when groundwater is absorbed by plant roots, carried up to the leaves and released back into the air.
Cool air causes the water vapour to condense into lots of tiny droplets to form clouds. When the clouds get heavy, precipitation occurs as rain or snow. Most of the water that falls to earth through precipitation stays in the ground - slowly moving through the aquifers and eventually forming streambeds, springs and oceans, ready for the cycle to begin again. (An aquifer is a layer of rock, which can absorb a large quantity of water; it can do so because it is porous or because the rock is broken up with crevices that hold the water.)
About two-thirds of the Earth's surface is covered in water - some 97% of this occurs as salt water in the seas and oceans, more than half of the remaining fresh water exists as glaciers and ice caps, and a very small amount of fresh water occurs as lakes, rivers and groundwater. Groundwater accounts for 0.6% of the distribution of water on Earth.