Susan Hoffman, age 12, of Lethbridge, Alta., Canada, for her question:

IS A PLATYPUS A BIRD, A REPTILE OR A MAMMAL?

The duckbill platypus of Australia is one of the oddest animals in the world. This, no doubt, is why we hear strange rumors about him. Some of these reports are misleading; some are downright wrong. This seems unnecessary because the facts about him are almost unbelievable.

His snout looks very much like a duck's bill, which is why he is called the duckbill platypus. He has webbed feet, somewhat like those of a goose, and the female platypus lays eggs. These features, one might think, are enough to qualify the platypus for membership in the bird world. Not at all.

Only birds have feathers. And the platypus wears a super‑soft fur coat, with some extra‑long hairs to shed the water. He is a warmblooded animal‑‑and all the reptiles are coldblooded creatures. What's more, the snakes, lizards and crocodiles wear scaly skins and turtles wear shells. None of the reptiles wears fur.

So the mysterious platypus cannot qualify either as a bird or reptile. Surely he must be a mammal because only the mammals wear hairy coats of fur, fleece or bristles. On the other hand, the mammals bear live babies‑‑and the female platypus lays eggs. This brings us close to the proper solution, which depends upon the animal's method of reproduction.

The female lays her eggs in secrecy, sealed inside her burrow. True, they are round, soft‑shelled eggs somewhat like those of the reptiles. But unlike the reptiles she nestles her eggs to her furry body until they hatch. What happens next disqualifies her from any relationship to the reptile clan.

The mother platypus feeds her babies on mother's milk. This is the one indisputable feature that qualifies her as a mammal. We can forget her bird‑type bill and her reptile‑type eggs. For only the mammals feed their babes on mother's milk.

Now let's take a closer look at her questionable features. Her bill is made of leathery material. A duck's bill is hard and stiff. Her two tiny white eggs hatch in a week to 10 days, and the blind babes are quite helpless. Baby reptiles are well developed and able to take care of themselves 'soon after they hatch.

The platypus is classed as one of the world's two egg‑laying mammals. The other is the spiny anteater, who also belongs to Australia. Nowadays they are strange indeed. But scientists suspect that such animals were common about 100 million years ago. At that time, most of the early mammals may have layed eggs and fed their helpless babes on mother's milk.