o knock him
over, and no amount of chopping makes so much as a lasting dent in the
tree.

Suppose you step in some mud. The mud does not stick to your shoes. It
bends down under your weight, but springs back to form again as soon
as your weight is removed.

And if you try to spread some elastic butter on bread, nothing will
make the butter stay spread. The instant you remove your knife, the
butter rolls up again into the same kind of lump it was in before.

As for chewing your bread, you might as well try to chew a rubber
band. You force your jaws open, and they snap back on the bread all
right; then they spring open again, and snap back and keep this up
automatically until you make them stop. But for all this vigorous
chewing your bread looks as if it had never been touched by a tooth.

Sewing is about as difficult. The thread springs into a coil in the
shape of the spool. No hem stays turned; the cloth you try to sew
springs into its original folds in a most exasperating manner.

On the whole, a perfectly elastic world would be a hopeless one to
live in.

_Elasticity is the tendency of a thing to go back to its original
shape or size whenever it is forced into a different shape or size._

A thing does not have to be soft to be elastic. Steel is very elastic;
that is why good springs are almost always made of steel. Glass
is elastic; you know how you can bounce a glass marble. Rubber is
elastic, too. Air is elastic in a different way; it does not go back
to its original shape, since it has no shape, but if it has been
compressed and the pressure is removed it immediately expands again;
so a football or any such thing filled with air is decidedly elastic.
That is why automobile and bicycle tires are filled with air; it makes
the best possible "springs."

Balls bounce because they are elastic. When a ball strikes the ground,
it is pushed out of shape. Since it is elastic it tries immediately to
come back to its former shape, and so pushes out against the ground.
This gives it such a push upward that it flies back to your hand.

Sometimes people confuse elasticity with action and reaction. But the
differences between them are very clear. Action and reaction happen at
the same time; your body goes up at the same time that you pull down
on a bar to chin yourself; while in elasticity a thing moves first one
way, then the other; you throw a ball down, _then_ it comes back up to
you. Another difference is that in ac