Now take a piece of writing paper and roll it into a tube and I'll show you something else. Stand the roll up between your two hands and press down on the top. It takes a good deal of pressure to bend or break it, doesn't it? Now lay it on its side and squeeze. It breaks right away. But how should the crystals in a piece of granite know that a column of anything will stand so much more weight when the pressure comes on the ends than when it comes on the sides? They seem to know; for I'll tell you what they do, away down there in the dark of the earth. The crystals stand at right angles to the pressure on the rock in which they are forming. Sometimes, because of the movements of the earth as it shrinks and cracks, the crystals already formed in granite are crushed over on their sides. Then, in course of time, they form again, but _this_ time they stand upright, with their "heads and shoulders" against the burden--little Atlases supporting the world! And they not only manage to get up and stand up straight when re-formed under pressure, but they stand closer together than they did before; they close up ranks, like soldiers with serious business before them. A crystal is made up of molecules, that is to say, little parts of itself. You can't see a molecule; you just have to think it. Each different thing in the world--as salt and sugar, boys and bumble-bees, little girls and butterflies--is made up of its own kind of molecules or little parts of itself. In order to grasp the idea of certain scientific facts, the men of science thought of the molecules themselves as being made of little bits of _themselves_, which the scientists called "atoms." Now they find that it is necessary--in order to work out still further their ideas of how things are made and done and changed, in this wonderful mystery we call the world--to imagine these atoms as made up of what they call "electrons." You mustn't think, however, that this is all mere fancy. We can, of course, think of anything as made up of small particles or parts of itself which we can call "molecules," and that these molecules are made of still smaller parts which we can call "atoms." But there is reason to believe that while each different kind of thing is made of its own kind of molecules and their atoms, all the atoms are made of the same thing--electrons or little bits of electricity. For reasons which need not be gone into here, it is known that electrons actually exist. These electrons are so much smaller than an atom that there is as much room for them to move around in an atom as there is for the planets to move around the sun. And they _do_ move--travelling round and round. There are, even in so small a thing as a grain of sand, untold numbers of these circling worlds; systems like the sun with its planets and other vast star systems of the sky.