MOTHER: Some birds and animals have more heat in their bodies than we do. The horse has one hundred degrees, the ox one hundred and one, the dog one hundred and two, the sheep one hundred and four, and the duck and pigeon have one hundred and eight. The bodies of some creatures, such as fishes and frogs, are much cooler than our own, and we call them cold-blooded. The frog has only seventy degrees of heat. [Illustration: _Fever thermometer._] HELEN: But what makes us warm, mother? MOTHER: Do you remember that we talked a good deal about our food as fuel not long ago? PERCY: But, mother, fuel is something to _burn_, and there is no fire inside of us. MOTHER: That is true in one way; but let us see if we can find out where the heat in our bodies comes from. It may be a little hard to understand, but we will try. Here is a candle. If lighted, it burns brightly. Now I will fasten a wire around it and lower it into this glass jar and cover it tightly. Now watch it. What is the matter? [Illustration: “_Now watch it._”] AMY: It is going out. Now it just flickers and hardly burns at all. Why does it go out, mother? MOTHER: Because all fire must have a part of the air called oxygen to make it burn. When the candle can have plenty of air, it burns brightly, but when shut up closely, where it soon uses all the oxygen, it will not burn at all. Now our bodies are much like the candle. We eat food, and when it is made into blood, it mixes with the oxygen we breathe, and as it goes round and round in the body, it makes heat. The difference between us and the candle is that the burning does not go on as fast in our bodies as in the candle, so there is no flame, and it would take much longer to make the same amount of heat. If you throw a piece of fat into the fire, it will burn. If you eat the fat, it will make just as much heat in your body, but it will last a long time. PERCY: How queer to think we are burning, bit by bit, just like a candle! MOTHER: Yes; just as long as we live, the fire is kept going. AMY: But I shouldn’t think that blood going around with oxygen in it would keep us warm. MOTHER: If that was the only way to heat the body, it would not. Where it is very cold, some houses have a grate; there may also be a furnace, and perhaps a stove besides. So there are three ways of heating the house we live in. The first, as I have told you, is by the blood carrying oxygen to every part of the body. That is like the grate. We will call the liver the furnace. We have found that all the starch and sweet things we eat are changed into liver sugar, and it is supposed this is used in the lungs to keep the body warm. HELEN: In what other way is the house heated? ELMER: I think I know. It is by exercise. When I run or play ball I become very warm. [Illustration: Frogs]