Jim Schillinger, age 12, of Santa Maria, California, for his question:

How does a mouse differ from a rodent?

This interesting question takes us into the neat topic called animal classification. We share our world with perhaps a million different ani¬mals and naturally we need an organized system to sort them out in our minds. For this purpose, scientists depend on an internationally known system of animal classification. According to this established set of rules, every little mouse IS a rodent    but not every rodent is a mouse.

The furry little scaly tailed house mouse, a's we know, is timid enough to keep out of sight and bold enough to invade our dwellings. To scientists he is a small rodent named Mus musculus. They explain that he originated in Asia, probably in India.

From there, he traveled by boat and caravan throughout Europe and he reached the New World as a stowaway on the early sailing ships. He is but one of hundreds of mice species that populate all parts of the liveable world. His first cousins include the deer mouse and wispy harvest mouse, the scrappy grasshopper mouse who looks as if somebody snipped off half his tail, the athletic kangaroo mouse and the pocket mouse who pockets food in his furry cheeks. The long list includes gerbils and jerboas, voles and hamsters and lemmings.

All of these mousy midgets are classified among some 6,400 rodents of the animal Order Rodentia. All rodents are air breathing, warm blooded mammals who give birth to live young. Their special incisors qualify them as members of the rodent clan. These long front teeth are chisels designed for gnawing.

Mice are small rodents, but it's not fair to give them all a bad name because some of the larger rodents are unpopular rats. Besides, the list of larger rodents also includes the sprightly squirrels and the gentle guinea pigs, the lazy woodchucks and porcupines, and the busy beavers.

There are differences, such as porcupine prickles and flat beaver tails but basically all the large and small rodents are very much alike. Their lives are dictated by the special teeth that set them apart from all other mammals. Most of them are nervous, hungry animals, always on the go. Most of them have very short life spans and almost all of them multi¬ply at a great rate.

The large order of rodents is subdivided into dozens of families to separate such groups as squirrels and beavers. The families are divided into small genus groups of close cousins    and there are hundreds of these rodent genera. The members of a genus are separate species. The scientific name of each rodent includes his genus, spelled with a capital letter, and his species, spelled with a small letter, such as Mus musculus, the house mouse.