Jamie had a small accident the other day. He was playing shortstop in a softball game, and as he tried to catch a ball, he used the hand without a mitt. He made the catch -- but he also cracked the bone in his thumb.

Jamie's parents took him to the emergency room, where he had his hand put in a cast. "You won't be able to play softball for a couple of weeks," the doctor told him.

At home, Jamie discovered that playing softball wasn't the only thing he wouldn't be able to do while his thumb was taped up. He couldn't do dishes -- but that didn't bother him too much. He couldn't tie his shoes. His couldn't practice the piano. And he couldn't write or draw, because he's right-handed, and that's the hand he broke.

Jamie started his summer science camp a few days after his accident. He told Sue, his counselor, about how he broke his thumb, and he explained that he was having trouble doing some things. "Until this happened, I never knew how much I use my thumb," Jamie told Sue.

"At least you can still use your left hand. It would be even worse if you couldn't use either of your thumbs," she said.

The next day, Jamie and the other campers in Sue's group tried an experiment.

"Jamie's broken hand gave me this idea," Sue said. "We're going to find out what life is like without thumbs."

Before the experiment began, Sue explained that human beings have a particularly useful kind of thumb. It's called opposable, which means that we can move it separately from our fingers, and move it across our palms to meet each fingertip. Only human beings, monkeys and apes have this kind of thumb.

Our opposable thumbs make our hands into amazingly useful tools. We can use them to do very precise work, like painting a water-color picture or mending a watch. The thumb also gives us a strong grip, so we can do things like throw baseballs with one hand.

The human hand is an amazingly complicated piece of equipment. Sue told the kids that each hand contains 27 separate bones. They're connected by joints that let the bones move. The bumpy knuckles on the back of the hand are some of these joints. Other joints allow the separate parts of the fingers to move.

A complex network of 38 muscles attached to the bones causes the hands to move. Some muscles control very fine movements -- like the touch of a concert pianist. Other muscles can grip hard, allowing someone to swing a baseball bat and hit a home run.

The human hand is also designed for grasping. With thumb and fingertip, a person can perform a delicate job like threading a needle. This type of hold is called a precision grip. Using a power grip -- with fingers and thumb curled around an object resting on the palm -- gives extra hold for tough jobs like hammering.

"Scientists are still trying to design robots that can move their mechanical hands as well as we use our human hands." Sue told the campers. "But they're discovering that the human hand is so complicated that it's very hard to imitate."

Sue wrapped white adhesive tape around each camper's hands. She taped their thumbs down, but left their fingers free. Then she set the group loose to try out various activities around the camp.

Some of the "thumbless" campers headed for the computer room. It wasn't too hard to use the keyboard, but they found that their hands got tired very quickly.

Other campers tried to play badminton, but they discovered that it's hard to grip a racket when you don't have a thumb. It's also difficult to throw the birdie in the air accurately. Another group of campers went to the snack bar, but they found it almost impossible to hold the ice cream cones they had bought. The campers who went to the pool for a swim decided to go wading instead after they found out how hard it is to unbutton your shirt to put on your bathing suit.

After an hour of getting along without thumbs, the campers were glad when Sue cut the tape off and freed them. Jamie only got one back for a while, since his other thumb was still healing. But he decided that one thumb was better than none. Tips for Parents

Is there a left-handed child in your family? It's not unlikely; some 10 percent of the U.S. population are lefties. Today's left-handed youngsters no longer have their hands tied behind their backs to force them to learn to write with the right hand. But there are still some difficult moments with wristwatches, desks at school, and scissors designed for the right-handed majority. Remind your child that he or she is in good company. Leonardo da Vinci was left handed. So is Robert Redford. Jimmy Connors plays tennis left-handed. If this isn't enough comfort, consider a membership in Left-Handers International, P.O. Box 8249 Topeka, Kan. 66608; (913) 234-2177. Membership is $15 per year. The group publishes a newsletter about famous lefties, does research about left-handedness, and provides an up-to-date catalogue of specially designed tools and equipment.

Catherine O'Neill is a contributing editor to the Health section.