Water is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical compound whose molecules are composed of two hydrogen atoms and one oxygen atom. It covers about 71 percent of the earth's surface, and relatively small amounts also occur as groundwater, in the polar ice caps and glaciers, and in the atmosphere as vapor, clouds and precipitation. More than 97 percent of the earth's water is saltwater, and only about two percent is fresh water. Water moves continually through the water cycle of evaporation, transpiration, condensation, precipitation, and runoff, mostly reaching the oceans.
Water is vital for all known forms of life, and is the main constituent of the fluids in living organisms, in which it acts as a solvent. However, it provides neither food nor energy. The human body consists of about 60 percent water and commonly cited figures for the water content of the human brain range from 73 to 85 percent.