Compressing a gas increases not only its density but also
its temperature. Conversely, when a gas is allowed to expand,
it cools off. Many everyday examples illustrate this behavior.
Pump up a bicycle tire, and the pump becomes hot because the
air is being compressed. Hold down the nozzle on an aerosol
can, and it feels cold because the propellant gas is expanding.
An air conditioner works by alternately compressing a gas to
make it hot, letting that gas cool, and then allowing the gas
to expand and get really cold. Every time we talk in this book
about an object made of gas, think back to the picture of atoms
or molecules bouncing around in a box, and remember the
commonsense ideas that explain the behavior of the gas.