beats down
with its wings against the air to force itself up; why you
push back on the water with your oars to make a rowboat go
forward.

INFERENCE EXERCISE

Explain the following:

101. Water comes up city pipes into your kitchen.

102. When you try to push a heavy trunk, your feet slip out
from under you and slide in the opposite direction.

103. When you turn a bottle of water upside down with a small
piece of cardboard laid over its mouth, the water stays in the
bottle.

104. You can squeeze a thing very tightly in a vise.

105. There is a water game called "log rolling"; two men stand
on a log floating in the water and roll the log around
with their feet, each one trying to make the other lose his
balance. Explain why the log rolls backward when the man
apparently runs forward.

106. The oil which fills up the spaces between the parts of
a duck's feathers keeps the duck from getting wet when a hen
would be soaked.

107. Sleds run on snow more easily than wagons do.

108. In coasting down a hill, it is difficult to stop at the
bottom.

109. When you light a pinwheel, the wheel whirls around as the
powder burns, and the sparks fly off in all directions.

110. You cannot lift yourself by your own boot straps.

SECTION 14. _Elasticity._

What makes a ball bounce?

How does a springboard help you dive?

Why are automobile and bicycle tires filled with air?

Suppose there were a man who was perfectly elastic, and who made
everything he touched perfectly elastic. Fortunately there is no such
person, but suppose an elastic man _did_ exist:

He walks with a spring and a bound; his feet bounce up like rubber
balls each time they strike the earth; his legs snap back into place
after each step as if pulled by a spring. If he stumbles and falls to
the ground, he bounces back up into the air without a scar. (You see,
his skin springs back into shape even if it is scratched, so that a
scratch instantly heals.) And he bounces on and on forever without
stopping.

Suppose you, seeing his plight, try to stop him. Since we are
pretending that he makes everything he touches elastic, the instant
you touch him you bounce helplessly away in the opposite direction.

You may think your clothes will be wrinkled by all this bouncing
about, but since we are imagining that you have caught the elastic