Charles Elgard, age 10, of Peterborough, Ontario, Canada, for his question:

What makes the center of the earth so hot?

Our scientists cannot dig down and explore the very center of our solid globe to see exactly what goes on. They can only deduce the conditions down there from in¬direct evidence. They get a lot of their information from mines and deep well drilling. They get more evidence from shuddering, world wide vibrations caused by earthquakes. Although even the best of their evidence is second hand and very indirect, scientists are clever enough to use it to figure out some of the earth's deep secrets. We know that things get hotter as we go deeper. Scientists have several ideas that explain why this may be so. But at present, we cannot explain all the details.

We know that crushing weight and pressure tend to warm up certain substances. We know that friction from jostling causes heat. The crust of the earth is immensely heavy and its restless slabs of rocks push and jostle each other. This pressure and friction certainly cause much of the underground heat. Radioactive substances also add heat. All these factors help to make the center of the earth hot, but most experts suspect that other unknown forces are working to make it as hot as it seems to be.