Carl Walker, age 9, of Santa Maria, California, for his question:

How does a refrigerator cool Coke inside a bottle?

We expect a refrigerator to cool the glassy outside of a bottle. But in a short while its chilly air reaches through the solid glass and chills the Coke inside. This is somewhat mysterious    until you recall what happens on a frosty day. That is, if you sit outside in flimsy clothes. When you first step outdoors, your skin shivers with chilly goose bumps. If you stay for a while, the frosty air bites deeper and soon you feel chilled to your very bones.

Heat and cold play the fastest, fanciest ball game in the world. The rule of the game is give and take and it lasts until all the players get an equal share of all the warmth in the neighborhood. The players are solid and liquid and gaseous substances, all made of miniature molecules. The heat is the energy they use in their frisky frolics. When molecules get more heat, they use it to jog and dash around faster. When they lose heat energy, they have to slow down. These changes make things feel warmer or cooler.

There is not much of a ball game when everything in a room is the same temperature. But things perk up when you light a fire, which adds a pocket of extra heat. Molecules near the blaze use its extra heat energy to move faster. They bash into slower molecules farther afield. This speeds up the slowpokes and they bash into more of their neighbors. The heat spreads as more and more players enter the game.

When the fire burns brightly, the heat spreads evenly through everything in the room. When it goes out, the game continues until everything has an equal share of the warmth. Then, if the day is frosty, the indoor heat begins to spread outdoors through cracks and even through solid walls. This is why a warm room soon grows cold when we turn off the furnace.

This also is why a bottle loses its inside and outside warmth inside a refrigerator, right through its solid glass. When it went inside, it was as warm as the room. Its jogging molecules began bashing the chilly slowpokes in the colder refrigerator air. Its warmth spread out from the glass, then from deeper and deeper inside.

The heat that the bottle lost added a tiny bit of warmth to the inside of the refrigerator. And the refrigerator must stay cool enough to chill the next Coke that arrives. It is chilled and rechilled by a special chemical, sealed into pipes inside its walls. This Freon chemical steals away warmth as it changes back and forth from a liquid to a gas.

The refrigerator has a built in compressor and condenser to change the Freon back and forth. It squeezes the gas into a hot liquid and lets it evaporate. The liquid Freon uses up a lot of heat to change back to gas    and it takes this heat from inside the refrigerator. It took away the warmth from your Coke and made the air inside chilly enough to cool the next one.