Water is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance (at least it should be), which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent). It is vital for all known forms of life, even though it provides no calories or organic nutrients.

Water covers approximately 70.9% of the Earth's surface, mostly in seas and oceans. Small portions of water are found in other sources like lakes, rivers, underground, and in the air as water vapor. Water moves continually through the water (hydrological) cycle of transpiration, evaporation, condensation, precipitation, and runoff, reaching the Earth’s rivers, lakes, oceans.