eed sticks to the bottom of your straw, the
straw flattens out when you suck.

8. When you pull your straw out to remove the seed, there is
no hole left in the lemonade; it closes right in after the
straw.

9. If you drop the seed, it falls to the floor.

10. If you tip the glass to drink the lemonade, the surface
of the lemonade does not tip with the glass, but remains
horizontal.

SECTION 4. _Sinking and floating: Displacement._

What keeps a balloon up?

What makes an iceberg float?

Why does cork float on the water and why do heavier substances
sink?

If iron sinks, why do iron ships not sink?

Again let us imagine ourselves up in the place where gravitation has
no effect. Suppose we lay a nail on the surface of a bowl of water. It
stays there and does not sink. This does not seem at all surprising,
of course, since the nail no longer has weight. But when we put a cork
in the midst of the water, it stays there instead of floating to the
surface. This seems peculiar, because the less a thing weighs the more
easily it floats. So when the cork weighs nothing at all, it seems
that it should float better than ever. Of course there is some
difficulty in deciding whether it ought to float toward the part of
the water nearest the floor or toward the part nearest the ceiling,
since there is no up or down; but one would think that it ought
somehow to get to the outside of the water and not stay exactly in the
middle. If put on the outside, however, it stays there as well.

A toy balloon, in the same way, will not go toward either the ceiling
or the floor, but just stays where it is put, no matter how light a
gas it is filled with.

The explanation is as follows: For an object to float on the water or
in the air, the water or air must be heavier than the object. It is
the water or air being pulled under the object by gravity, that pushes
it up. Therefore, if the air and water themselves weighed nothing, of
course they would be no heavier than the balloon or the cork; the
air or water would then not be pulled in under the balloon or cork by
gravity, and so would not push them up, or aside.

[Illustration: FIG. 11. The battleship is made of steel, yet it does
not sink.]

WHY IRON SHIPS FLOAT. When people first talked about building iron
ships, others laughed at them. "Iron sinks," they said, "and your
boats will go to the bottom of the sea." If the boats wer