?H ? O, composed of two elements hydrogen and oxygen. Under the atmospheric pressure is colorless transparent liquid, is the source of human life.

?Water, including the natural water (rivers, lakes, atmosphere, water, sea water, groundwater, etc.), the artificial water system (through chemical reaction for hydrogen atom in combination with water).

?Water is one of the most common substance on earth, all life, including inorganic compound, human survival of the important resources, is also the most important part of the organism. Water played an important role in the evolution of life. It is a kind of special non-renewable resources, generalized renewable resources.