If you were to take water (like many other materials) and break it up into almost the smallest things you could, you’d get molecules. If the molecules are stuck together really tightly in a regular pattern, then they’re called a solid. The solid form of water is ice. This actually makes a lot of sense, because it certainly does seem like all the little parts of a solid (like ice) are stuck together very tightly.
When you heat something up, it makes the molecules move faster. If you heat up a solid, it melts and becomes a liquid. In a liquid (like water), the molecules are still stuck together, but they can move around some. What actually happens is that the molecules are still sort of sticking together, but they’re constantly breaking apart and sticking to different molecules. This also makes sense when you think about water. Water sort of sticks together, but it breaks apart /really/ easily.
If you heat a liquid like water up even more (like if you put it in a pot on the stove), then the molecules will move around so fast that they can’t even hold on to each other at all. When this happens, all of the molecules go flying apart and become a gas (like when you boil water to make steam). The process of gas molecules leaving the liquid to go into the gas is called "evaporation." The opposite process is called "condensation."
Hope this answers your question!
-Tamara (small mods by mbw)
(republished on 07/24/06)