Birds have no teeth, yet they eat hard seeds, like acorns and grains of corn. To break these up, and get them ready for the stomach, they have a gizzard, which is a sort of grinding-mill. And to help in the work of grinding they swallow small stones. One of the wonderful things about birds is the height at which they can live, and not only live, but fly. A man cannot go higher than twenty-two or twenty-three thousand feet, while moving about or exercising, because the air is so rare he cannot breathe. The highest a man was ever known to go and live, it is said, was less than thirty thousand feet, and that was in a balloon, where he did not move. But birds go a good deal higher than this, and can fly--which is violent exercise--at that height. It is thought by some that the thinness of the air may be the cause of the great speed with which birds fly in that region. But there is still much to be found out about this. Besides the marvels of flight, birds have other powers almost as strange. Many of them can fly under water with perfect ease, and, more than that, they can, when they wish, sink slowly till nothing is left above water but their beaks, to breathe. And they can stay so as long as they choose, keeping still in one spot, without moving. A cormorant in a zoölogical garden, who wanted to catch some of the swallows skimming over the pond, sank his body till only his head was out, and held himself there perfectly still. Birds who are hunted, as geese, have been known to save their lives in that way, by sinking their body under water, leaving in sight only the tip of the bill, which is so small it is not readily seen. To do such things, birds must be able to make their bodies heavy when they choose, as well as light, which we know they are able to do by filling their air-sacs with air. There are many things still to be found out about the powers of birds. XXII HIS BEAK AND TONGUE HOW does a bird get along without a hand? He has to prepare food; to keep his feathers in order; to build the nest; to feed and take care of the young; and sometimes to fight other birds. How can all this be done without a hand? [Illustration: FIG. 1. Bill of Oriole.] The beak is the only thing most birds have in place of a hand, and it is wonderful to see how many things they can do with it. Orioles use it as a needle, in making the nest. With it they weave strips of soft bark or strings, back and forth, in and out, to make the firm pocket they hang on the elm-tree (see Fig. 1). [Illustration: FIG. 2. Bill of Woodpecker.] A woodpecker's beak is a chisel or pick, to cut a deep hole in a tree trunk for a nest (Fig. 2). With a nuthatch it is a hammer, to crack the nut he has wedged into a crevice in the bark so tightly it cannot slip. [Illustration: WHITE-BREASTED NUTHATCH]