If you blow on your fingers, you feel that your breath is much warmer than the air. If people are crowded together in rooms with doors and windows shut, their breath soon heats and poisons the air, until they begin to have headache, and to feel dull and drowsy and uncomfortable. If they should be shut in too long, without any opening to let in the fresh air, as in a prison cell, or in the hold of a ship during a storm, the air would become so poisonous as to make them ill, and would even suffocate them and kill them outright. Even the bees found this out thousands of years ago; and in their hives in hot weather they station lines of worker-bees, one just behind another from the door right down each of the main passages, whose business it is to do nothing but keep their wings whirring rapidly, so that they fan a steady current of fresh air into every part of the hive. [Illustration: DUSTING--HOW SHALL WE DO IT?] How does Mother Nature get rid of these poisons from our breath? Of course, you say, "She uses the wind and the sunshine." Yes, the winds can whisk up the poison and blow it away so fast, and the sunshine can burn up the horrid smell so quickly, that even the air above big cities, and in their streets, is quite clean enough for us to breathe, except where the people are very closely crowded together and very dirty. Mother Nature wants all of us to help in keeping the air clean. This we can do by keeping ourselves and our houses clean, and by being careful not to leave scraps of waste, or dirty things, in the streets and cars and parks and other public places. And you children ought to be very careful about your school yard and the halls and the classrooms, where you spend so much of your time. IN SCHOOL I. BRINGING THE FRESH AIR IN The only place where air is absolutely sure to be fresh is out of doors. There, as we have seen, the sun and the winds keep it so all the time. But, unluckily, we cannot spend all our time outdoors, either when we are little or after we have grown up. So we must try in every way that we can to bring the outdoors indoors--to get plenty of fresh air and light into the houses that we live in, especially the bedrooms we sleep in and the schoolrooms we study in when we are children, and the offices or shops we work in when we are grown up. After you have your lungs and your blood well filled with air, either by walking briskly to school or by chasing one another about the school playground, you will suddenly hear the bell ring, and you march indoors and sit down at your desks. Here, of course, the air cannot blow about freely from every direction, because the walls and doors and windows are shutting you in on every side. The room, to be sure, is full of air; but if the doors and windows are shut, this air has no way of getting outside, nor can the fresh, pure air out of doors--even though it be moving quite fast, as a wind or a breeze--get inside.