ightly swollen
if they swing them while taking a long walk.

96. A flywheel keeps an engine going between the strokes of
the piston.

97. In dry parts of the country farmers break up the surface
of the soil frequently, as less water comes up to the surface
through pulverized soil than would come through the fine pores
of caked earth.

98. After you have apparently cleaned a grease spot out of
a suit it often reappears when you have worn the suit a few
days.

99. Mud flies up from the back wheel of a boy's bicycle when
he rides along a wet street.

100. A typewriter key goes down less than an inch, yet the
type bar goes up nearly 5 inches.

SECTION 13. _Action and reaction._

How can a bird fly? What makes it stay up in the air?

What makes a gun kick?

Why do you sink when you stop swimming?

Whenever anything moves, it pushes something else in an opposite
direction. When you row a boat you can notice this; you see the oars
pushing the water backward to push the boat forward. Also, when you
shoot a bullet forward you can feel the gun kick backward; or when
you pull down hard enough on a bar, your body rises up and you
chin yourself. But the law is just as true for things which are not
noticeable. When you walk, your feet push back against the earth; and
if the earth were not so enormous and you so small, and if no one else
were pushing in the opposite direction, you would see the earth spin
back a little for each step you took forward, just as the big ball
that a performing bear stands on turns backward as the bear tries to
walk forward.

[Illustration: FIG. 38. The horse goes forward by pushing backward on
the earth with his feet.]

The usual way of saying this is, "Action and reaction are equal and
opposite." If you climb a rope, the upward movement of your body is
the action; but you have to pull down on the rope to lift your body
up. This is the reaction.

Without this law of action and reaction no fish could swim, no
steamboat could push its way across the water, no bird could fly, no
train or machine of any kind could move forward or backward, no man or
animal could walk or crawl. The whole world of living things would be
utterly paralyzed.

[Illustration: FIG. 39. As he starts to toss the ball up, will he
weigh more or less?]

When _anything_ starts to move, it does so by pushing on something
else. When your arms start to