Ok, so Judy Garland didn't exactly ask that in the heartwarming "Mister Wizard of Oz." It's still an interesting question. Why are there three states of matter anyway? As a small child you learned it as a fact, but now we ask, why so?

Of course, there are all manner of answers possible, such as stories of magic beans or the long ago Three Brothers and their Gifts, or whatever. Three is always a popular number. (There's an exercise: write a just-so story about a physics topic! If you have a good one, send it to us. Here's a teacher mixing creative writing with astronomy teaching.) However, this being a physics page, we'll stick to physics answers.

The point is not just that matter can be thick or thin, like soup, but that there are jumps between states. If you slowly heat or cool a substance, its properties change by a little by little smoothly for awhile, but then as the temperature reaches a certain value (depending on the substance) the properties abruptly jump to something quite different: ice melts or water vapor condenses or whatever. So the question is: why are there jumps, and why specifically three states and not more or fewer?

There isn't a really simple answer (in fact, part of it won a Nobel Prize!) but it can be understood in a fairly intuitive way. We'll go slowly.

The first step is to divide the problem into two. (Did you know there are two kinds of people: those who divide things into two and those who don't? ;)

The key idea is that a substance is made of molecules, tiny bits all the same. We group the states into two categories: solids and fluids.

(Naturally, later on we're going to split up fluids into liquids and gases)

The usual visualization goes something like this: imagine a dozen small hard candies in a pie pan. If you gently shake the pan, keeping it nearly horizontal, the candies will slide around, occasionally bouncing off each other or the walls, but having no pattern. Now if instead you put the candies one each into the pockets of an ice cube tray and shake that, the candies still bounce around but only in their pocket: they retain their average regular grid positions relative to each other. Of course, the pan is supposed to be a fluid and the ice cube tray a solid.

What's not so clear in the analogy is what the walls of the ice cube tray correspond to. They seem to be something apart from the candies which controls the candy behavior, whereas in the case of solids all we have are molecules.

The answer: when two molecules get close to each other, each feels a force from the other, like the familiar "static electricity" attracting or repelling little bits of paper or whatever. The inter-molecular force is attractive at first, as the molecules approach each other, causing them to move closer than they would otherwise. However, as they become very close, the force changes to strongly repulsive, pushing the molecules apart. (again, a creative writing exercise! ;)

If the molecules are moving around very fast, when they occasionally hit each other, they just bounce off like billiard balls, because their speed carries them so close to each other they feel the repulsive force and are pushed apart. This is a fluid.

However, if the molecules are moving slowly enough, they can "stick" together with the attractive force. They can still move, rattling in the "potential well" (a real physics jargon term which will make physicists believe you know what you're talking about) like candy in the ice cube tray, but they stay fairly fixed relative to each other. When enough molecules stick together to notice, you have a solid.

Molecules in a real substance travel at many different speeds, but the average speed is characteristic of the state of the substance: it gives the temperature! A substance when hot has faster moving molecules than when cold. So a solid at a certain low temperature has its molecules moving (vibrating) at a certain low average speed, not enough to escape the sticky force. If you heat it up, raising the temperature, you increase the speeds until eventually the molecules start escaping and moving off free on their own. This is known by the highly technical term "melting." (Completely unlike what happened to the Wicked Witch of the West) You can also make a solid from a fluid by increasing the pressure, jamming the molecules close together in a smaller volume so that they stick even though they're moving fast, because they can't get very far from one molecule before reaching another.

Here's a nice interactive Java animation of sticky molecules.

Kitchen pressure cookers are an example of how pressure changes things: there's liquid water inside, even at temperatures above the "normal" boiling point of 212° F or 100° C. (did you know that Colonel Sanders' original pressure cooker is enshrined at the KFC Museum?) Inversely, at lower pressures like on mountains at high elevations, water boils at lower temperatures. For humans in space vacuum it's more interesting: an accident during a NASA test of a space suit in a vacuum chamber on earth caused a man to be exposed to vacuum long enough to pass out for lack of oxygen. He later said the last thing he remembered was the saliva boiling on his tongue. Less dramatic but still cool classroom demo1 and demo2.

That was easy

OK, that's solids and fluids. What about solids, liquids and gases? Ah, now it gets a little trickier...this is where the Nobel Prize comes in. Why should fluids have sharply defined states like liquid and gas? What happens at evaporation and condensation if the molecules in both liquid and gas are moving around randomly, bouncing off each other? To find out,

follow the yellow brick road.

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