Dumb question!

[Silverthorn]

<physics with a dash of chemistry>

To fully understand the difference between a liquid and a solid, you'll have to look at it at the atomic level. To make it all nice and smooth, we look at the water-molecule H₂O (and not some weird tap water which has minerals and stuff dissolved in it):


The fat sphere is the oxygen, the smaller ones are hydrogen. Now, what does this tell us about its state? Nothing

It is about the connection between the individual molecules that defines the form. If you have single H₂O-molecules floating freely around, with practically no interaction between each other, you have a gas (aka. water vapor).

Push these things a little closer together so that weak bonds can form. You'll get a terrible mess of molecules that are oriented in any way. They somehow stick together but not really strongly. That's liquid water.

The temperature is directly linked to the speed these molecules move along. The lower it is, the slower the molecules. In gas-phase, they are so fast that no bound can form but the colder it gets, the more bonds are there that are not broken up again.

At cool temperatures, most stuff crystallizes into some sort of regular shape¹⁾. For water/ice, it kind of looks like this:



You basically get some symmetry into the water. According to thermodynamics, things want to end up as chaotic as possible, so this ordered structure is actually kind of against it (but it has the best energetic conditions of the individual molecules, so there is some kind of equilibrium). Long story short, the ordered condition can be seen somewhere else, too:



Looks pretty ordered, too. In fact, snow flakes are little water-crystals. Therefore, they're solid.

Aerated water, on the other hand, is just liquid water where you slap some bubbles into. You wouldn't want to call that "gas", would you?

¹⁾ Quantum Mechanics disallows a state of complete rest. This means, stuff has a zero-point-energy which prevents Helium from crystallizing, unless you force it into some really awkward conditions.

</physics with a dash of chemistry>

And yay, Blue Phoenix's longposts strike once again!