To see why solids melt and liquids freeze, you need to know two things about matter (solids, liquids, and gases). First, all ordinary matter is made up of atoms, often attached together as molecules. The matter tends to stay together because those molecules are pulling on each other. The other thing is that molecules are always bumping and moving around, and the hotter they are, the more they move.
When you get a solid hot, the molecules bump around faster and harder until they start breaking apart and moving around each other; that's when a solid melts and becomes a liquid. However, in a liquid, the molecules still pull on each other enough to stay bunched together (thatís why a drop of water on a table is round, and doesnít spread out all over). If you heat the liquid up even more, the molecules fly around fast enough to break totally free of each other; now they can go anywhere, and the liquid boils off into a gas and just spreads out and mixes into the air.
The opposite happens when you cool down a gas: first it becomes a liquid, then a solid.
Some solids can even become a gas without first becoming a liquid; this is called "sublimation". For instance, ice in a freezer slowly disappears, since itís subliming into a gas. The opposite happens, too: thatís why you get frost (which is tiny ice crystals) in a freezer, and on cold window panes.
Changing the pressure on a solid, liquid, or gas can make it change state, too.
(republished on 07/25/06)