Fig. 6 —Photograph of the outer dead skin pushed off from a black snake crawling through the brush.
Why the Body needs Food.—Every living thing, whether a plant or an animal, needs food. While the whole body lives, a part of it is constantly dying. The entire outer layer of a snake's skin dies three or four times during a year and is cast off, sometimes in a single piece. We can scrape dead bits of skin from the surface of our body at any time. Tiny particles are dying in all regions of the body, and we should soon waste away if food were not taken to make up the loss for the worn-out parts.
The body also needs food to help it do its work and keep warm. The body has the strange power of using food eaten to make the legs and arms move and the brain to think. In doing this the body is said to burn the food.
How the Body burns itself and also Food.—If a boy is weighed just before playing a game of ball and again afterward, he will find that part of his body has been used up and given off in the breath and sweat. He has burned part of his body, and the breath and sweat are like the smoke given off when a match is burned.
One fifth of the air is made of a gas called oxygen. When anything becomes very hot, this oxygen makes it burst into a flame and burn. We breathe in oxygen with the air and the living action of the body causes such a slow union of the oxygen and the tissues that there is no blaze although there is a little heat.
Kinds of Food.—There are four general classes of foods. These are the building foods, the sugars and starches, the fats, and the mineral foods. The building foods are those which help largely in forming new muscle and blood or other parts of the body. Proteids is another name for building foods.
Sugars and starches are placed in one group because starch changes to sugar within the body. If you chew a starchy food like bread for a few minutes, it will begin to taste sweet because the starch is becoming sugar.
Fats are got not only from fat meat but also from eggs, butter, milk, and many other foods. There is some mineral matter, such as potash and soda, in many of the vegetables and meats eaten, and we use much table salt to season other foods.