You could almost guess what each of these is for, just by looking at it. The nose and the ears are open and hollow because air must pass into them in order to bring us odors or sounds; while the eyes are solid, somewhat like big glass marbles, to receive light--because light can go right through anything that is transparent. Eyes, ears, and nose all began on the surface, and sank gradually into the head, so as to be surrounded and protected, leaving just opening enough at the surface to allow smells, light-rays, and sound-waves to enter; and all of them have at their bottom, or deepest part, a sensitive patch of surface, which catches the light, or the smells, or the sounds, and sends them by a special nerve to the brain. These three sets of organs have gradually and slowly grown into the shape in which we now find them, in order to do the particular kind of smelling, seeing, and hearing that will be most useful to us. Every kind of animal has a slightly different shape and arrangement of eye, of ear, and of nose to fit his particular "business"; but in all animals they are built upon the same simple, general plan. THE NOSE How the Nose is Made. The nose began as a pair of little puckers, or dimples, just above the mouth, containing cells that were particularly good smellers, in order to test the food before it was eaten. All smells rise, so these cells were right on the spot for their particular "business." The original way of breathing, before the nose-dimples or pits opened through into the throat, was through the mouth; and that is one reason why it is so easy to fall into the bad habit of mouth-breathing whenever the nose gets blocked by _adenoids_ or _catarrh_. Some creatures--fishes, for instance,--breathe through their mouths entirely; if you watch one in an aquarium or a clear stream, you will easily see that it is going "gulp, gulp, gulp" constantly. The saying "to drink like a fish" is a slander upon an innocent creature; for what it is really doing is breathing, not drinking. Even a frog, which has nostrils opening into its throat, still has to swallow its air in gulps, as you can see by watching its throat when it is sitting quietly. And, strange as it may seem, if you prop its mouth open, it will suffocate, because it can no longer gulp down air.[28]