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