John Hamlett, age 12, Tuscon, Arizona, for his question:

 Why can't anything get colder than minus 273 degrees?

There seems no limit to how hot a substance can get. The face of the sun is a seething 6,000 degrees Centigrade. The heart of the sun may be 20 million degrees or much morel The centers of some stars are even hotter. But nothing in the whole Universe can be colder than minus 273.2 Centigrade. On the Fahrenheit scale this coldest of cold is minus 459.8 degrees. We call this the temperature of Absolute Zero.

In order to understand cold, we must understand a little about heat. Heat puzzled the scientists for centuries. At one time they thought it was a kind of fluid. Nowadays we attribute heat to busy molecules. Molecules are those small particles of matter from which everything is made. A couple of billions of them could frolic on the head of a pin.

All the solids, liquids and gases in the world are composed of molecules. And all the molecules we know about are busy and on the move. For molecules are separate from each other and free to rush about, which they do at a great rate. Molecules of solids, say iron or wood, are more closely packed together. However, they too are always restless and probably move by vibrating. Molecules which form liquids tumble around each other as though they were dancing a hoedown with linked hands.

It is these restless molecules which give heat to a substance. The faster they move, the more heat the substance has. As a substance loses heat, its molecules are merely slowing down. It gains heat again as its molecules gather speed and dash about in all directions.

At 0 degrees Centigrade, the molecules of water slow down and form solid ice. At 100 degrees they speed up and fly off as vapor and join the other gases in the air. The molecules of iron take on solid form at normal everyday temperatures. At 1,530 degrees Centigrade they break apart to form a seething liquid. Helium gas does not get into a liquid state until it reaches minus 269 degrees Centigrade.

Each substance has its own freezing point. This is the point at which its molecules slow down to form a solid. This does not mean that the molecules then come to a dead standstill. Molecules may be shivering a little, even in a chunk of ice.

Absolute zero occurs when all movement among the molecules comes to a dead stop. There is then no heat whatever in the substance.

The vast reaches of space between the stars may be near Absolute Zero. On our cozy little planet, nothing in nature ever gets anywhere near thin point. Scientists use the Kelvin, or Absolute temperature scale when trying to reach Absolute Zero in the laboratory. So far they have not quite achieved it ‑ though they have less than one degree to go.

The degrees on the Absolute scale are equal to the degrees on the Centigrade scale. However, 0 degrees Centigrade is the freezing point of water. The 0 on the Absolute scale is Absolute Zero ‑ which is as cold as anything can possibly get. For at that bitter temperature, the atoms and molecules of every substance come to a dead halt.