And that, it is thought, may be one of the secrets of the continual change of things; clay rock changing to granite, granite to soil, soil to fruit, fruit to children, and so on--everything on the move and the electrons doing the moving--carrying the changes, so to speak--these wonderful little myriad messenger boys of the universe! HIDE AND SEEK IN THE LIBRARY Don't imagine, for all I've talked so long about them, that I've told you everything there is to know about the crystal fairies. For example, did you know that if it wasn't for the crystal people we wouldn't have any ice? (_Ice._) You will also find that if it wasn't for ice--ice and the Greeks--we wouldn't have the word "crystal" at all. (_Crystal._) One of the most striking things in the whole conduct of these clever crystal folks you will find in reading about ice. If it wasn't for a peculiar--a very peculiar--habit the ice crystals have, all the waters of the world that ever freeze at all, would freeze solid to the bottom and never _would_ thaw out! I'll tell you this much about it: While everything else in the world--including boys and girls--contracts when it gets cold, ice expands, and so becomes lighter than water, and so floats. And yet the ice crystals know how to contract as well as expand, and that's why ice sometimes builds stone walls, as we will see when we come to study "The Stones of the Field" in July. Shaking still water that is cold enough to freeze but hasn't frozen makes the crystal fairies get very busy in their ice factories. And it looks very much as if the fairies themselves warmed up with their work; for, after this shaking, the temperature of the water rises ten degrees at the very same time it is freezing! You will also find that when the weather is cold enough ice itself freezes, gets harder and harder with the cold; that ice will melt ice; that two blocks of ice will grow into one if you give them a chance; that ice crystals are apt to be born twins; that these twin crystals are fond of gardening--at least, they raise "ice flowers"; that the ice crystals are so punctual in their coming and going in water that they are used to help place the markings on thermometers just right, so that we can tell exactly how cold or hot we are.