Kathy Haas, age 12, of Bethlehem, Pennsylvania, for her question:

What makes a jumping bean jump?

This year, as usual, the jumping bean has baffled a new generation of human children. Nobody expects a bean to go bouncing around like a bronco. Nobody expects a vegetable to make any sort of move on its own. But at certain times of the year, remarkable, three cornered beans are sold in novelty shops in the United States, Canada and possibly other countries. And no doubt about it, these jumping beans do jump    at least for a while.

This is a two part story that begins in Central or South America. It involved a plant of the Euphorbiaceae, or spurge, family and an insect named Laspeyresia saltitans. The spurge is related to the Christmas red poinsettia and also to the dainty snow on the mountains that grows in shady gardens. Most spurges have milky white sap, and some of the tropical species produce latex that can be used to make rubbery substances.

The spurges in our story are rather scrawny trees and shrubs that grow among the desert vegetation of semi tropical America. In spring they produce drab little flowers. You would hardly notice them    but they are noticed by the gypsy moth relative of our story. When the female is ready to lay her eggs, she seeks out these particular spurge blossoms. She is a small, drabbish grey moth, and you would hardly notice her either, 'as she flutters around placing an egg into one, then another and another drab little spurge blossom. When her duty is completed, she dies and departs from our story.

Soon the spurge petals fall and each moth egg sleeps safely inside a developing seed. When the egg hatches, the grubby larva finds itself inside a small edible bean. Naturally it eats away at the inside walls of its little prison. The bean and the grub grow together, and the growing grub hollows out the inside of the bean.

After a while, she feels rather cramped and in need of exercise. So she coils her caterpillar body like a spring    and lets go with a jerk. You can guess what happens. The moth larva is strong enough to move the whole bean. Every time she does her setting up exercises, the jumping bean jumps around like a miniature bronco.

Moth type insects, however, must change through four distinct stages, and each stage lasts only so long. When the jumping bean larva gorges her fill, she becomes drowsy and changes into her pupa stage. The pupa sleeps and the jumping bean's jumping days are over.

The pupa, of course, is not the final stage of life for the moth. While it sleeps, its body is metamorphized and remodeled. The creature that emerges is a winged adult, ready to mate and produce eggs that will start the next generation of mysterious jumping beans. The nibbling larva left a thin wall at one end of the bean. The newly hatched moth pushes her way through this spot and flutters forth to find those modest flowers that bloom on the right species of desert spurges.