As if everything in the desert were in the sand-making business the very rain-drops help make sand. The rain-drops do this in much the same way that the farmer breaks big boulders in his fields, so that he can more easily haul them away, piece by piece. He builds a fire against the boulder, gets it as hot as he can, then rakes the fire away, dashes water on the stone, and--bang! It cracks as if old Thor had struck it with his hammer. You see why this is, don't you, after what we have been saying about why the rock's skin chips off? The water suddenly cools the highly heated rock, and the parts shrinking pull away from each other with a bang! bang! bang! The hot desert rocks, dashed by the torrents of a cloudburst, break apart just like that, and you can hear them. Stones twenty-five feet across are often broken into many pieces after a downpour. Then the finer pieces of rock that are made in this continual splitting, and by the chipping that goes on day and night, the fierce winds grind against each other; so manufacturing sand. And the fiercer winds also drive coarse sand against crumbling rock surfaces, thus grinding them away and making more sand. So the winds, using sand to make sand, put the sand out at interest, you may say. And on all its sand, made in these various ways--by wind and rain and heat and cold, and the crystal fairies of the land of change--the desert puts its special trade-mark, just as a manufacturer puts his trade-mark on his goods. If you should take some desert sand and some sand from the shores of the sea and show them to a man who knows about such things, he would say (after he had put them under a microscope, of course): THE DESERT'S TRADE-MARK ON ITS SANDS "_This_ sand came from a desert, or from some place where it was much blown about by the winds; while _this_ sand is from the shores of the sea, or of a lake." The sand grains of the seashore, although they are always being tumbled about by the waves, as the desert sands are by the winds, are protected from each other by the water between them. These little water cushions prevent the sand grains from rubbing together; so they keep a good many of their sharp edges. They are not rounded like the sands of the desert. The winds keep the desert sands grinding against each other, at the same time turning them over and over, so wearing them away pretty evenly on all sides. It also grinds them against the desert rocks. [Illustration: A DESERT SIMOOM ON ITS TRAVELS A traveller in the Sahara took this snap-shot of a simoom from the outside and at a safe distance. You can see that it must be quite a distance from where we are standing, for the trees in the foreground are still. The vast cloud of sand looks quite dark because of the shadows cast by the sun, which it hides from view. ]