beats in your wrist or in your neck.

Does your heart ever become tired? Not while you keep well, unless you
over-drive it by running or wrestling too hard. It can rest between
the beats. But the heart muscle, like any other muscle, must have
plenty of good red blood to feed on. You put food into the blood by
eating good breakfasts and dinners. The more you run and jump and
play, the more work the heart has to do and the stronger it grows; and
a good morning romp before school will send the blood flowing so
merrily round from top to toe that you will feel fresher and brighter
all the day.


III. FRESH AIR--WHY WE NEED IT

The heart is not the only thing that goes faster and harder when you
run about in the morning and play hard. You are breathing faster and
deeper as well, as if there were something in the air outside that you
needed in your body as much as food.

But, of course, you know that air is not good to eat. It has no
strength in it, as food has; it isn't even a liquid like milk or
coffee or tea. It is so thin and light that we call it a _gas_.
Indeed, I suppose it is pretty hard for you to believe that air is a
real thing at all. But all outdoors is full of the gas called air, and
everything that seems to be empty, like a room or an empty box, is
full of it.

You cannot even smell it, as you can that other gas which comes
through pipes into our houses and burns at the gas jets; nor can you