You can see the flame when the candle is burning, but you can't see the fires that burn in our bodies; there are no real flames at all. I know it is hard for you to believe that there can be any burning when our bodies are so wet and damp. But if you can't see it, you can easily feel it. Blow on your hand. How warm your breath is! Touch your hand to your cheek. It is quite warm, too. If you run or play hard, you sometimes become so hot that you want to take off your coat. That is because your fires are burning faster. The muscles are using more food and making more scraps to be burned. You breathe faster and faster till at last you are "out of breath" and feel as if you would smother or choke. The blood has hard work to bring oxygen enough to keep the fires going. After the cells have burned the food scraps, they turn the "ashes" and "smoke" back into the blood-stream that is always flowing past them. If the cells did not do this, they would soon smother to death, just as you could not possibly live in a house without chimneys to carry off the smoke. And, of course, the blood wants to get rid of this waste just as quickly as possible. Part of the waste in the body is liquid, like water, and can flow away through the blood pipes without needing to be burned. Some of this watery waste comes out through the skin and stands in beads or drops upon it. That is the part we call perspiration, or sweat. The rest of it goes in the blood to another strainer called the _kidneys_, passes through this as _urine_, and is carried away from the body as the waste water from the bathtub and the sink is carried away from a house. For the "smoke" Mother Nature has still another beautiful plan. She sends the blood-stream flowing through the _lungs_, where it can send off its "smoke" and then get fresh air to carry to the cells in the muscles. When you breathe out, you are sending out the "smoke"; and when you breathe in, you are taking in fresh air. Our body "smoke" is not brown or blue, like the smoke from a fire; it is a clear, odorless gas, called _carbon dioxid_. This is the same gas that makes the choke-damp of coal mines, which suffocates the miners if the mine is not well ventilated; and the same gas that sometimes gathers at the bottom of a well, making it dangerous for anyone to go down into the well to clean it. And this gas is poisonous in our bodies just as it is in the mine or the well. You see, then, how important it is that we should live much of our lives in the clear pure air out of doors, and should bring the fresh air into our houses and schools and shops. "Fill up" with it all you can on your way to school, for the best of air indoors is never half so good as the free-blowing breezes outside. IV. FRESH AIR--HOW WE BREATHE IT