In its purest form, it's odourless, nearly colourless and tasteless. It's in your body, the food you eat and the beverages you drink. You use it to clean yourself, your clothes and your dishes. You can travel on it or jump in it to cool off.

All forms of life need it, and if they don't get enough of it, they die. Political disputes have centred on it. In some places, it's incredibly difficult to get. In others, it's incredibly easy to get and then squandered. What substance is more necessary to our existence than any other? Water.

Two of water's unique properties are: cohesion and adhesion. Cohesion refers to the fact that water sticks to itself very easily. Adhesion means that water also sticks very well to other things, which is why it spreads out in a thin film on certain surfaces, like glass. When water comes into contact with these surfaces, the adhesive forces are stronger than the cohesive forces. Instead of sticking together in a ball, it spreads out.

Water is often known as the universal solvent, which means that many substances dissolve in it. Substances that dissolve in water are hydrophilic. This means that they are as strong or stronger than water's cohesive forces. Salt and sugar are both polar, like water, so they dissolve very well in it.

Substances that do not dissolve in water are hydrophobic. This is the source of the saying "oil and water don't mix." Water's solvency is why the water that we use is rarely pure; it usually has several minerals dissolved in it.

The presence of these minerals is the difference between hard water and soft water. Hard water
usually contains a lot of calcium and magnesium, but may also contain metals. Soap will not lather well in hard water, but hard water isn't usually dangerous. It can also cause lime scale deposits in pipes, water heaters and toilets.

Some of the latest controversy about water's properties lies in how ice behaves when it melts. Some scientists claim that it looks about the same as it does when it's solid, except that some of its hydrogen bonds are broken. Others claim that it forms an entirely new structure. So, for all of its importance, we still don't completely understand water.