Suppose we looked at a piece of ice through the imaginary microscope that shows us the molecules. The ice molecules would be different from the iron molecules in size, but they would be vibrating back and forth in exactly the same way, only with less motion. It is because they have less motion that we say the ice is colder than the iron. Then let us suppose that the sun was shining on the ice while we watched the ice molecules. First we should see movements of the ice molecules become gradually more rapid, just as the iron molecules did when the iron was warmed. Then, as they moved faster and faster, they would begin to bump into each other and go around every which way, each molecule bumping first into one neighbor, then into another, and bouncing back in a new direction after each collision. This is what causes the ice to melt. When its molecules no longer go back and forth in the same path all the time, the ice no longer keeps its shape, and we call it water--a liquid. Almost all solid substances will melt when they are heated. Or, to put it the other way around, every liquid will freeze solid if it gets cold enough. Even liquid air (which is ordinary air cooled and compressed until it runs like water) can be frozen into a solid chunk. Some things will melt while they are still very cold; solid air, for instance, melts at a temperature that would freeze you into an icicle before you could count ten. Other things, such as stones, are melted only by terrific heat. When the little particles of water that make up the clouds become very cold, they freeze as they gather and so make snowflakes. When the little particles of water in the air, that usually make dew, freeze while they are gathering on a blade of grass, we call it frost. When raindrops are carried up into colder, higher air while they are forming, they freeze and turn to hail. When snow or frost or hail or ice is heated, it melts and turns back to water. [Illustration: FIG. 47. Why did the bottle break when the water in it turned to ice?] But here is a strange fact: although heat usually expands things, water expands when it _freezes_. Like everything else, however, water also expands when it becomes hot, as you found when you made a kind of thermometer, using a flask of water and a glass tube. But if you should put that flask into a freezing mixture of ice and salt, you would find that when the water became very cold it would begin to expand a little immediately before it froze.