Water, in its purest form of one oxygen atom and two hydrogen atoms, in other words H2O for the technically-minded, is a transparent, odourless, tasteless liquid. It's apparently quite important since it can make up to three quarters of a human's bodyweight and accounts for 70.7% of the surface area of the Earth.
Water freezes at 0°C or 32°F and boils at 100°C (212°F) at sea-level1, and has a couple of useful properties:
Water has a high heat capacity - It can absorb and release large amounts of energy before its temperature changes appreciably.
Water is an excellent solvent - Though it is not the universal solvent, as it is sometimes claimed to be, it can easily dissolve small, reactive chemicals such as salts, acids and bases2 into an evenly distributed solution.
A curious property of water becomes evident when you freeze it. In general, as you freeze something, its volume decreases due to the reduction of kinetic energy of the atoms. In the case of water, however, its frozen state - ice - is actually greater in volume due to the crystalline arrangement of its atoms taking up more space. This means that ice floats on water, unlike many solids which plainly do not float on their liquid counterparts.
Uses for Water
Water is a useful substance, but due to its range of uses, you may not be getting the most out of it. To help you, here is a list of some of the things you can do with water:
You can bathe in it.
You can squirt it at fires to put them out3.
You can squirt it at people for entertainment.
You can freeze it to put in drinks.
You can use it as a mixer in drinks.
If you have enough frozen water you can make an Igloo with it.
You can shave the frozen water to make the base substance for an ice water drink.
You can bottle it to sell to people.
You can boil it to produce steam to give enough pressure to drive a locomotive, or produce a cute whistle on your kettle.
You can swim in it.
You can warm it up and sit in it while a pump blows bubbles of air through it - also known as a jacuzzi.
You can sweat in it in a steam room or a sauna.
You can dissolve things in it to produce different foods (for example, jelly) or drinks.
You can dissolve things in it (eg, soap or detergent) to produce a mixture to clean other things.
You can dissolve things in it to use on an oil burner for aromatherapy purposes, or to rid the house of nasty smells.
You can squirt it from small nozzles to produce impressive displays.
You can fill it with carbon-dioxide to produce Soda Water to use as a mixer in drinks.
You can put minerals in it to produce... er, mineral water.
You can spray it on things to cool them down.
You can cut things with it if you spray it fast enough.
You can run it through turbines to produce electricity.
You can run from it when it starts to rain.
You can swear at it when you've just put the clothes out to dry and it begins to fall from the sky.
You can make plants grow with it4.
You can use it as part of a hangover cure.
In extreme emergencies, you can drink it.
This is a type of water where the hydrogen atoms (usually with just a single proton in the nucleus) are replaced by deuterium atoms (one proton and one neutron in the nucleus). It's used chiefly as a source of deuterons for experiments in Nuclear Physics.