One would think that the edges of the teeth, at any rate, must soon be worn away. Nature has guarded against this danger by making these teeth of two different substances. The face of the tooth is made of a very thin plate of hard enamel, the rest of the tooth of much softer bone. During use, of course, the soft bone is worn away very much faster than the hard enamel, and so the sharp, cutting edge is preserved. It is interesting to find that we make our chisels in a very similar way. The blade is not a solid piece of steel, of the same quality throughout; it consists of steel of two different qualities. The face of the tool is a very thin plate of extremely hard steel, but the rest is of much softer metal. And as it is with the rodent's tooth, so it is with the chisel. The soft metal is worn away during use much faster than the hard, so that the edge is not destroyed. Only two pairs of front teeth are developed in the rodent animals, and as the "eye" teeth are wanting there is always a gap in each jaw between these and the grinders. THE COMMON SQUIRREL First on our list of rodent animals comes the common red squirrel, which of course you know by sight very well. There are very few parts of the country where we may not see it frisking and gamboling among the branches of the trees, or sitting upright on its hind quarters and nibbling away at a nut, which is delicately held between its front paws. It skips up the trunk of a tree quite as easily as it runs along the ground. That is because its sharp little claws enter the bark, and give it a firm foothold. And it scarcely ever falls from a branch because its big bushy tail acts as a kind of balancing-pole, like that of a man walking upon a tight rope; and by stretching it straight out behind its body, and turning it a little bit to one side or a little bit to the other, the animal can nearly always manage to save itself from a tumble. Even if it does fall, however, it does not hurt itself, for the skin of the lower part of the body is very loose, and it is fastened for a little distance along the inner surface of each leg. So, when the animal falls from a height, it merely stretches out its limbs at right angles to its body--stretching out the loose skin, of course, with them--and so turns itself into a kind of open umbrella, just like the parachutes which are often sent down from balloons. And instead of tumbling headlong to the ground and being killed by the fall, it is buoyed up by the air and floats down comparatively slowly, so that it is not hurt in the least.