Margaret Cardozo, age 14, of Grand Forks, N.D., for her question:

WHY DID THE DINOSAURS DIE OUT?

Dinosaur is the common name for a group of reptiles that lived on earth millions of years ago. Scientists have given us a number of reasons why they believe the creatures died out and became extinct.

One theory is that a giant meteoritic impacted on the earth causing tremendous dust that covered the planet for many months blocking the sun and causing plants to die.  Most dinosaurs were plant eating animals.  When the plants were gone the dinosaurs slowly disappeared.

Probably the main cause of the disappearance of the dinosaurs was the rise of mountain ranges that were formed during the Cretaceous period of the earth's history. When the mountain ranges formed, the great seaways drained from the continents and the vast swamplands that were the homes of the dinosaurs dried up.

When the swamps dried up, tremendous changes in the climate and food supply developed. The dinosaurs had become specially adapted to the old conditions. When these conditions changed rapidly, the dinosaurs couldn't adjust.

As conditions on earth changed, the plant eating dinosaurs discovered that they could not live on the new plants. As the plant eaters died out, the meat eaters that had depended on them for food also died.

The disappearance of the dinosaur actually was a long, slow process that took between 10 million and 20 million years. But by the end of the Cretaceous period, all of the dinosaurs had disappeared from the earth.

Scientists tell us that other factors, including disease, probably affected the dinosaurs. Other scientists say that the development of mammals also ranked as an important factor in the extinction of the dinosaurs.

Mammals have better brains than the dinosaurs and better protection against cold. Some of the mammals probably ate dinosaur eggs and young dinosaurs. Although they were not the only cause, they almost certainly helped in the extinction of the great reptiles.

The word dinosaur comes from the Greek words dinos, meaning terrible, and sauros, meaning lizard.

Dinosaurs were not lizards, but the biggest flesh eating dinosaurs certainly earned the word "terrible" to describe them. They stood as much as 20 feet high and had skulls four feet long, with huge dagger like teeth. The biggest plant eaters were up to 90 feet long and some weighed as much as 85 tons. The plant eaters were definitely "terrible," too.

Dinosaurs dominated the world during the Mesozoic era which ran from about 225 million years ago to about 65 million years ago.

Scientists learn about dinosaurs from fossils, the records and remains of ancient living things. A scientist doing this work is called a paleontologist.

Scientists believe that large dinosaurs lived to be more than 100 years old.

Although many dinosaurs were huge, some were very small. One type was only two and a half feet in length.

Dinosaurs ruled the earth for about 145 million years, the scientists say.