Water, technically Hydrogen oxide (H2O), is a compound of hydrogen - the universe's most abundant element - and oxygen - it's third most abundant - is the most abundant substance in the universe which is stable as neutral particles and contains two or more elements. (The only more abundant diatomic species are H2 and HHe+.)(1) It is common throughout the universe except where temperatures are too high or pressures too great.

Water is abundant on Earth, but far less so than in space. Earth's core is water-fee, as is the inner 75% (by radius) of its mantle. Water of hydration is expected only in the upper 660 km of the mantle. Although some calculations indicate there may be several times as much water in earth's mantle as its oceans, this water is not free. Free water has been found at depths exceeding 100 km, however, in the form of ice VII crystals trapped in diamond.

Only near earth's surface does water become ubiquitous. It occurs as a solid on the poles and during cold seasons; it occurs as a liquid in oceans, rivers, and soil; and it occurs as a gas in the atmosphere. It is, by an order of magnitude, the most important solvent in all chemical processes taking place near the earth's surface.

Earth has a water cycle that makes sure its water doesn't get lost. Most of Earth's water, however is "salt water". This means there is some sodium chloride dissolved into it. Only 3% of Earth's water are "fresh water" which humans drink to survive.

Most water is made with the Protium isotope of Hydrogen, however there also is heavy water: Water with Deuterium instead of Protium. This type of water tastes somewhat sweet, but should not be consumed in large amounts. Water with the radioactive Tritium isotope, "superheavy water", should never be consumed, even in small amounts, because it is strongly radioactive and will increase the weight of your body more drastically than heavy water.

Water is also the most common solvent in chemistry: many acids and bases are diluted with it. Despite water being neutral, it is quite corrosive; it can rapidly oxidize most metals and darkens surfaces.