=Remember.= 1. Take care of your teeth and they will last you as long as you live. 2. Do not pick them with pins, or toothpicks of any kind. 3. Do not use them for nutcrackers or wire-cutters. 4. Do not use them for tack pullers. 5. Keep them clean at all times. 6. Brush them up and down as well as crosswise. CHAPTER XI NECESSITY FOR PURE AIR AND HOW TO SECURE IT We have learned how the cells of the body are killed by starvation. Now let us learn how they are choked to death, or killed by lack of air. [Sidenote: How air is changed in the body] The cells of the body need _oxygen_, and the only way we can give it to them is by means of air. Every time we take air into our lungs we are giving oxygen to the red corpuscles or cells in the blood, which distribute it to the other cells in the body. The air that goes into our lungs, if it is fresh and pure, contains a great deal of oxygen and a very little of another gas called _carbon dioxid_. The air that comes out of the lungs contains a very little oxygen and a great deal of carbon dioxid. The blood not only takes the oxygen out of the air, but gives carbon dioxid to the air. This carbon dioxid is very poisonous, and would kill the cells if it remained in the blood; hence we should never breathe the same air twice. There is no lack of fresh air in the world, and no excuse for anyone's ever breathing air that is not pure. [Sidenote: Effects of impure air] If you close all the windows and doors in the schoolroom and shut up the ventilators, you will soon find that you are not able to pay close attention to your studies, and in a little while you will begin to feel drowsy. This is because you have used up so much of the oxygen in the air that there is no longer enough to supply the demands of the little cells, and because, in addition, you are taking into your bodies the poisonous carbon dioxid that has been breathed out into the room. It takes a great deal of fresh air to supply the body with oxygen--about 1,250 cubic feet of air each hour. With thirty or forty children in a room, it does not take long to use up all the oxygen. So there should be a constant supply of fresh air coming into the room. [Illustration: FIG. 25. Results of breathing good and bad air.] [Sidenote: Methods of ventilation]