Bob Scott, age 13, of Annapolis, Md., for his question:

JUST WHAT IS A MARSUPIAL?

A marsupial is any mammal that has a pouch on its underside. The name comes from the Greek word "marsupion," which means "pouch." The pouches are on the female mammals, which use them to carry their young.

Opossums and kangaroos are the two most famous marsupials.

Newborn marsupials are tiny and helpless. A mother opossum may have as many as 18 babies, each about the size of a bee. As soon as they are born, the babies crawl into the pouch and attach themselves to nipples inside. At first the babies are so small that they can't even suckle. The mother must pump milk into their mouths.

Fossils show that marsupials are one of the oldest groups of mammals. Once they were found in all parts of the world.

Marsupials vary in size from tiny, mole like, creatures that are just a few inches long, to the kangaroo, which can stand almost seven feet tail.

Today most of the world's marsupials are found in Australia and nearby islands. The opossums of the Americas are the only exceptions.