Andrea Friday, age 11, of Montreal, Quebec Canada for her question:

How do alligators and crocodiles differ?

Alligators and crocodiles are toothy reptiles that tend to bite any person or animal that comes within range. For this reason, it is wise to leave this sorting job to the experts. We ordinary folk should observe them only from a safe distance—or study their portraits. If one of the long, flat scaly giants has his mouth closed tight, chances are he is an alligator. If he wears a wicked grin on his toothy face, chances are he is a crocodile.

The animal order Crocodilia includes all the crocodiles and alliga¬tors and also their cousins the caimans and the gavials. All of them resemble prehistoric monsters of the bygone Age of Reptiles. As a matter of fact, we have found the fossilized remains of an ancestral crocodile that lived during the days of the dinosaurs. He was much like a modern crocodile, except that he was 50 feet long.

The main difference between modern alligators and crocodiles is in their toothy jaws. The crocodile’s snout is longer and more pointed. The alligator’s snout is shorter and more rounded at the tip. Both have wicked teeth in the upper and lower jaws. Both have some extra long teeth in the lower jaw. If this problem were not solved, neither would be able to close his big mouth. And the cousins solve this problem in different ways.

In the alligator’s upper jaw, there are pits directly above the extra long teeth in the lower jaw. When he closes his mouth, the long lower teeth fit neatly into the slots above. The alligator can close his big mouth—and his lips. The crocodile cannot do this. Instead of pits to park his long lower teeth, he has grooves along the outside of his upper jaw.  When he closes his mouth, the long lower teeth slide up into these grooves. But the fit is not perfect—and a section on each side of his upper lip is pulled out of shape. This is what creates that wicked looking smile on the face of the crocodile. Naturally this is not intended to be a ferocious grin. But it does help a non expert observer tell whether one of these animals is an alligator or a crocodile.

All of the squat, scaly crocodilians live in and around streams and marshes of tropical and semi tropical regions. Some live in coastal regions where the water is salty. Nowadays, many adult alligators and crocodiles grow to be 12 feet long and some of them weigh more than 400 pounds. Most of the e world’s crocodiles enjoy life in Africa and Asia, though some share the waters of Florida with our native alligators.

There are differences among the four major groups of crocodilians, though all are obviously related. The Asian gavials look like big croco¬diles with thin snouts and no smiles. The caimans of the American tropics range from four to 15 feet in length—and they may be very fierce. All crocodilians have plenty of sharp teeth. But the caimans and gavials do not have those extra long teeth in their jaws.