other way around, every liquid will freeze solid if it
gets cold enough. Even liquid air (which is ordinary air cooled and
compressed until it runs like water) can be frozen into a solid chunk.
Some things will melt while they are still very cold; solid air, for
instance, melts at a temperature that would freeze you into an icicle
before you could count ten. Other things, such as stones, are melted
only by terrific heat.

When the little particles of water that make up the clouds become
very cold, they freeze as they gather and so make snowflakes. When the
little particles of water in the air, that usually make dew, freeze
while they are gathering on a blade of grass, we call it frost.
When raindrops are carried up into colder, higher air while they are
forming, they freeze and turn to hail. When snow or frost or hail or
ice is heated, it melts and turns back to water.

[Illustration: FIG. 47. Why did the bottle break when the water in it
turned to ice?]

But here is a strange fact: although heat usually expands things,
water expands when it _freezes_. Like everything else, however, water
also expands when it becomes hot, as you found when you made a kind
of thermometer, using a flask of water and a glass tube. But if you
should put that flask into a freezing mixture of ice and salt, you
would find that when the water became very cold it would begin to
expand a little immediately before it froze.

And it is very lucky for us that water does expand when it freezes,
because if it did not, ice would be heavier than water is. But since
the water expands as it freezes, ice weighs less than water and
floats. And that is why lakes and oceans and rivers freeze over the
top and do not freeze at the bottom. If they froze from the bottom up,
as they would if the ice sank as it formed, every river and lake would
be solid ice in the winter. All the harbors outside the tropics would
probably be ice-bound all winter long. And the ice in the bottom of
the lakes and rivers and in the ocean would probably never melt.

So in the case of freezing water, and in the case of a couple of
metals, there is a point where coldness, not heat, makes things
expand.

EXPERIMENT 32. Take a ketchup bottle with a screw cap and a
cork that fits tightly. Fill it to the top with water; put
a long pin beside the cork while you insert it, so that the
water can be crowded out as the cork goes down; then when you
have pushed the cor