You see, when you run, you are working your muscles and heart-pump very hard; and if you kept running all the time, you would burn up so much food in the muscles that the heart couldn't pump blood fast enough to wash away all the waste, and would just chug-chug-chug till it tired itself out. When you are tired, it is time to stop and rest; for being tired means that the poisons are not being carried away from the muscles fast enough, and that your heart is working too hard. What is it in your body that gives it stiffening to stand upright, and makes levers in your legs and arms to move it about? When you feel your body and arms and head with your fingers, what are they like? Isn't there something hard and then a soft kind of pad over it? We call the hard things _bones_. Your teacher will show you some. These are white and chalky looking; but when they were alive, they were a beautiful pinkish white color. [Illustration: SKELETON OF A MAN] So you have a pretty pearl-colored framework, the shape of your body. This, which is called your _skeleton_, makes you stiff enough to stand up and walk about. Now bend your arm and turn your wrist and open and close your hand. You find that your frame-work is jointed. When you are tired standing, you can bend your joints and sit down. If you want an apple, you can close your fingers and pick it up. [Illustration: THE MUSCLES OF THE ARM] [Illustration: WHEN THE MUSCLES SHORTEN] What are the soft pads that you felt over the bones of your arms and legs? Stretch your right arm straight out in front of you and take hold of the upper part of it with your left hand. Now clench your right fist and bring it toward your shoulder. Can you feel the elastic pads, or bands, moving? What are they doing? They are pulling your hand up to your shoulder. When you walk, you can feel the elastic bands moving your legs along. So every move we make, these elastic ropes are at work pulling us about and letting us sit down and making us run and jump. We call them _muscles_. You have perhaps seen jointed dolls. The strings and rubber bands on their joints help to make them move; but the dolls don't act as if they were alive. They have no telephone system to tell their bodies how to move. If you will stop and think how many "moves" you make in a day, you'll know how hard your muscles have to work. They'd be quite tired out if they did not have plenty to feed on all the time and did not rest at least nine hours a day. I told you how the food is melted and carried about in the blood. It is the blood that brings the muscles their food and keeps them alive and makes them strong enough to move the joints and the bones. What does all this playing do for you? It makes you grow not only big, but strong, too. What puny little things you'd be if you couldn't get out and run and play and make your muscles strong and your nerves do just what you tell them to do.