Water is a compound consisting of two hydrogen atoms and one oxygen atom. It was once considered an element (a classical element, along with earth, fire, and air) and the compound that is very essential to life. Its chemical formula is H2O, and it makes up more than 2 1/3 of your body. Most liquid water on Earth forms part of the hydrosphere.
Water, mostly salt water and in the form of oceans, covers roughly 71% of the Earth's surface. It was long believed that carbonaceous asteroids (rather than comets due to their high deuterium content compared to Earth's water) brought water to Earth, but new evidence using molybdenum isotopes found that Earth's water originated on Theia, the planet which crashed into Earth to form the Moon.
Water is odorless, tasteless, nearly colorless and non-toxic. It is sometimes considered the universal solvent due to it's ability to dissolve almost all substances. Water is special because it's solid form (ice) can actually float. This is because when liquids are cooled to solids, they have less energy. This goes for water too, but it has two hydrogen atoms which repel each other. This helps to make solid water being less dense and actually floating on liquid water. However, this property is not unique, and is shared with Acetic Acid, Silicon, Gallium, Germanium, Antimony, Bismuth and Plutonium. Water is densest at 4 °C, having a density of 1 g/cm3. At higher temperatures, it is very slightly more dense; for example at 20 °C, water's density is 0.99821 g/cm3. Water can only take the form of three out of the four states of matter here on Earth, the planet in which water covers 71% of its surface.
Water has an obvious melting and freezing point of 0 °C and a boiling point of 100 °C.
Water is an acid because when it reacts with aqueous Hydrogen Fluoride, it donates a proton. And when it reacts with aqueous Ammonia, it donates a proton, meaning it is a base.