Karen Russell, age 10, of Visalia, Calif., for her question:

DOES DRY ICE OCCUR IN NATURE?

Dry ice is that frosty stuff we take along on a picnic to keep the cold drinks cold. It is cold enough to freeze flesh, and no sensible person touches it with a finger. It is colder than ordinary ice, and it does not melt and turn to water. As it gets warmer, it simply disappears into thin air.

When water chills to 32 degrees, it freezes and turns to solid ice. Lots of other substances freeze solid long before they get this cold. For example, the rocks of the earth's crust are frozen solid. Their private freezing temperatures are much higher than the freezing point of water. They remain frozen on seething hot days when the weather is as hot as it can get.

Every substance has its own freezing and melting temperature, which may be warmer or cooler than the weather. The air is made of substances that melt arid boil at everyday temperatures. One of the gases in the air is carbon dioxide, the waste gas we breathe out when we empty our lungs. The weather never gets cold enough to make it freeze solid.

Dry ice is made of carbon dioxide chilled to its freezing point. This colder‑than‑cold temperature is minus 112 degrees. If you are preparing for the arrival of the metric system, you can translate the freezing point of carbon dioxide to minus 80 degrees Celsius.

There may be cold planets far from the sun where the temperature drops this low. But it never happens in nature on the cozy planet earth. If it did, the gaseous carbon dioxide in the air would freeze solid‑‑and become dry ice. So naturally dry ice does not occur in nature, at least not on our planet.

This leaves us wondering how carbon dioxide can be chilled to such an unearthly temperature. Certainly refrigerators and freezers never get cold enough to freeze it. But there are other ways. First, the carbon dioxide gas is compressed or squeezed in a pressure tank. Strange to say, this makes it hot, but this is just the first step.

When the hot gas escapes through a valve, it spreads out and expands. .This makes it cooler. Time after time the carbon dioxide is compressed and allowed to expand‑‑and each time it gets colder.

Actually, the escaping expanding gas becomes cooler because the operation makes it give up some of its heat. As it loses heat, it gets colder. When it chills to the unnatural temperature of minus 80 degrees Celsius, it is 80 degrees colder than ordinary ice. It has reached its private freezing point‑colder than anything in nature. Then it changes from a gas into the frozen solid we call dry ice.