Daniel Powle, age 9, of Peoria, Illinois for his question:

How do we know prehistoric animals really lived?

Some of .us enjoy Monster Movies, even though they are a bit scary. We are told that Weird Creatures are visitors from other worlds. But naturally we know that these shows are really just made up stories. Then we hear that serious scientists actually believe in monsters. They tell us that Weird Creatures once lived right here on earth. Naturally this makes a young; person wonder. Are these prehistoric monsters also made up stories.

The people who make Monster Movies invent some amazing creatures. But the truth is that some of the prehistoric animals were even more amazing. The mightiest monsters were the dinosaurs. But for a long time people refused to believe that they ever really lived on the earth. So the scientists had to prove they did. They also proved that a lot of other strange animals, large and small, also lived here ages before human history began.

This unbelievable problem began when people kept finding strange old bones buried in the ground. Different animals have different type bones    and experts know all about them. These strange old bones could not have belonged to any animal of modern times. Some must have belonged to elephant sized animals. Other bones and whole skeletons must have belonged to reptiles, somewhat like giant lizards.

These buried remains are called "fossils." Researchers also found footprints of giant reptiles and even a few nests of their fossilized reptile eggs. These and other fossils proved that all sorts of strange animals really did live in prehistoric times. The next problem was to prove when they lived. And scientists found ways to prove this also.

For billions of years, the earth has been building new layers of crusty rocks. Experts know more or less when each layer was formed. Suppose a volcano erupted a million years ago and buried a big dinosaur under lava and ashes. Nothing disturbed the volcanic layer and in time the dinosaur bones became fossils. Suppose at last the skeleton was found, just where it had been buried. The rocks around it tell an expert that the fossilized dinosaur must have lived and died a million years ago.

Experts always study the rocks around a buried fossil. They also make clever tests with radioactive materials, such as carbon 14. This is radioactive dating and proves just about when a prehistoric animal lived.

This test is possible because uranium and other radioactive substances decay through the ages. One by one, like steady clockwork, their atoms change to different atoms. Small traces of radioactive materials are trapped in stones and old bones. Scientists measure to tell how long each tiny sample has been decaying. If it was trapped in a fossil bone, it tells how long ago the prehistoric animal lived on the earth.