We are, then, gas engines. So we have to have air to mix with our gasoline. The simpler water animals, such as sponges, which are mostly holes, and all minute creatures, both animals and plants, simply take it in directly into their cells where they are going to use it. There is plenty of air in water—you can see it fizzle out from the water in a drinking glass when you draw water from a faucet in cold weather. The water creatures breathe this out of the water, and die of suffocation if you put them in boiled water from which the boiling has driven out the air.

Most animals which have blood, use this to carry the air to their cells. For blood, whatever else it is, is nine-tenths water, and will dissolve air like any other water. The insects, however, though they have blood, do not use it to carry air. Instead, they have a system of branching pipes running all over their bodies, and opening at various points on the surface. You can often make these out easily, a pair of openings for each joint, on the sides of caterpillar’s body. These pipes carry the air everywhere over the insect’s body, even to the feet, so that wherever there is a working muscle, there also is the air for it to work with. Thus the insect has no need of lungs, and has none; and therefore, I suppose never gets out of breath, no matter how hard it works.

We human beings, and our four-footed cousins, all backboned animals in fact, do not manage in any of these ways. We breathe the air into our lungs. There, instead of dissolving it in the watery part of the blood, we turn it over to the red corpuscles, which are especially made to do this very thing and do it particularly well. These minute, coin-like corpuscles carry the air all over the body, and deliver it over to the cells as they need it. But of course, as you must have already learned in school, the body handles only the part of the air that it can use, the oxygen. The rest it lets go and doesn’t bother with. That is where we have the advantage over other automobiles, which can’t pick out the part they want but have to take the air as it comes. Still it all comes to the same thing in the end. With all animals the oxygen gets mixed with the fuel and explodes.

Our fuel, moreover, is a good deal like gasoline. Gasoline, as you know, is related to kerosene, benzine, paraffine, and the rest, which are all products of rock oil. They are, then, themselves oils; and gasoline is an oil.

We, too, eat oils; not, to be sure, mineral oils, but animal and vegetable oils, olive oil and butter and cream and all sorts of fats; for fats are merely oils that freeze at common living temperatures and melt only after we get them stowed away.

We, then, burn many sorts of oil. We also burn bread and potatoes and the like, starch and sugar and gums, which though not oils, are much like them; really in a way, oils that are already about half burned. These we finish up in our engines. On the whole, it’s much more convenient than depending on one sort of fuel, and exploding only gasoline.

I am not going to stop now to tell you the long story of how the bread and potatoes and the rest of our food finally gets changed over into a sort of sugar; and is as sugar, packed away in the cells of our muscles and other tissues, mixed with the oxygen of the air, and made ready to explode when the signal through the nerve touches it off. The food is taken apart and put together again, combined and separated, stored up when it isn’t needed, and used sometimes in one way and sometimes in another. Different animals treat their food differently after they get it swallowed; even different human beings, eating the same food, do not always handle it quite the same way.

Most of us take our food into our stomachs, but the earthworm crawls through the earth, and at the same time lets a stream of earth crawl through him, digesting what is food and leaving the rest behind as he moves along. Amoebas sometimes flow round little water plants many times longer than themselves, crawl along the stem, with the stem sticking out front and back, and digest the juices as they go along. The star-fish, which lives on oysters larger than himself, turns his stomach inside out, sticks it into the oyster’s shell; and after he has digested the oyster, pulls his stomach back again. A dog will digest bones; and a cow will digest wood; while a fish will swallow another fish nearly as long as himself, keep the tail, still unswallowed, in his mouth while he digests off the head, and than moves his meal up another notch.

There are all sorts of queer freaks, but the main point is that, in the end, all our food gets built into the cells of our bodies; much of it in the form of sugar, and that this sugar explodes as if it were the gasoline vapor in a gas engine that some man has made. With the force of these explosions, the body does its work; it keeps itself warm with the waste heat.