Water
Water is one of the Earth's basic naturally occurring substances. It covers about 70% of the world's surface, and is one of the key parts to the survival of species. Up to 60% of the human body is water.
The word "water" itself is practically synonymous with the word "liquid", as we refer to different liquids as water-like: "watered down", or "watery". We know that water moves and flows and is a force; to come across another liquid which visibly resembled water with an unknown chemical makeup, we might infer that it is water but would not know until more evidence was discovered.
Scientific makeup
On a molecular level, water is a chemical compound comprised of two atoms of hydrogen and one atom of oxygen (H2O). Water by definition is a liquid, although it can be converted to different states by the removal or addition of heat.
When heated to 100 degrees Celcius (its boiling point), water begins to convert to steam, and when cooled to 0 degrees Celcius (the freezing point) it converts to ice. Water is unique in that it is the only naturally occurring substance that is found in these three states.
Additionally, water is usually referred to as "the universal solvent" because of its ability to dissolve more substances than any other existing liquid. It has a neutral acidity, which on a pH scale has a measurement of 7.
Uses
The availability of water on the Earth affords humanity an incredible number of uses, aside from consumption as an integral part of survival. Water can be used to cool machinery and facilities, such as nuclear power plants and industrial milling tools. Water can also be heated to generate power--the focus of the steam engine which was born out of the industrial revolution, and hydroelectric dams which use water flow and gravity to turn turbines and rotors to generate electricity. Water can also be pressurized, creating a narrow stream that can cut through conrete and steel.