Water, as well as many other materials, if not all, can exist in different states: solid, liquid, and gaseous.

In a gas, molecules are far apart from each other. They are very fast (which means that the gas has a high enough temperature, since temperature measures the degree of agitation of molecules). They bump them, but do not stay together. A gas is compressible if you put in a piston is pressed, it will take less space. This is normal, there are plenty of free space left by the molecules. A gas is a just as people from each other and ran in all directions.

In the liquid state, molecules are "free": they go around, not stay. But they are tight because they attract each other. We must imagine that they will soon bump them, but they do not move away as much from each other: there is a liquid at a temperature lower than the gaseous state. They are a bit stuck between them, just because a molecule does not have enough speed to escape the attraction of others. But there is not much space between molecules. So a compressible liquid is difficult. We can do the same with so many people, but tight, this time. Each person is free to move, but remains stuck to the other necessarily, because of the pressure. If the crowd is "deformable" it is not "compressible".

The liquid and gas fluids: molecules are free in, ie not attached to their neighbors, liquids and gases are fully deformable.

Finally, there are solid state, at temperatures even lower. In ice, for example, water molecules are arranged: they are found in rows, and they remain in their place. If they hold together, precisely because they attract. But then, their speed is so low that the molecules that do remain in place: they can not escape the pull of their closest neighbors. The solid state is in fact a lens: the ice is a crystal, ie the molecules are arranged in a very precise and orderly, geometric. It's a bit this time as a group of people chained to each other: they can still move, agitated, but they do not have the freedom to change one's neighbors. Something funny even when it exists for the ice such as several kinds of ice - the molecules are not arranged in the same way in each of these crystals. There are ice heavier than water, for example, but it is not that found in normal, since we know that ice floats on water (iceberg).

Most materials are located under one of these states: gas, liquid or solid (crystal). But there are others: the glass, for example, is not a crystal, contrary to what one might believe. It has the properties of a solid, mechanically speaking, if you do not look too closely. In fact, the atoms of the glass are misplaced, they say it is an amorphous state. And in fact, the glass could sink if we were to apply a sufficient and for a very long time. Contrary to what one sometimes hears, it is not because the glass flows that old windows are irregular (just, they did not do well plate glass planes once).

Another example? Beneath our feet, it is often said that the mantle is liquid that is molten rock. It's true! That said, if you had a sample on hand, you could just attack the pneumatic drill is so hard. It sank, but very, very, very slowly, and under very tight constraints, such as those found in the mantle!

If one increases the temperature of a gas, it can turn into plasma: there's been so hot that the electrons are expelled atoms from impact. A plasma is a gas of electrons and atomic nuclei. It talking about the fourth state of matter, but there are on earth only rarely: when electric shocks (in the flash, what), or when trying to make nuclear fusion!

Finally, a little funny phenomenon: the surfusion. Changes of state are not always as expected. Water can be liquid below 0 ° C, without problem. But it is unstable: you can touch, and it solidifies. We used this phenomenon to make water bottles magic!