In 35 years as a General Foods research scientist, William Mitchell proved beyond the shadow of a Twinkie that necessity isn't the only mother of invention. Among the imitation fruits of his creativity: a tapioca substitute, carbonated ice, powdered alcohol (140 proof) and simulated fruit-crush desserts. But Mitchell, 67, may have reached the apex of his restless talent with the improbable invention of Pop Rocks: carbonated candy crystals that crackle on the tongue. The company has sold more than 500 million packets since introducing them in 1974. Mitchell himself is a hard-core enthusiast. "Oh man!" he exclaims. "It snaps, it fries—you don't know where it's coming from!"

Recently, however, speculation has turned in unwelcome directions. Rumors of an apocryphal child who devoured three sacks of Pop Rocks, drank a fizzy soft drink and promptly exploded have alarmed the Northeast. "There is no danger," scoffs Mitchell. "The worst thing the rocks could do is make you burp. The amount of gas in Pop Rocks is less than one-tenth the amount in a can of soda pop." But a nervous GF has launched a costly campaign to smite the canard. It has run full-page ads in 45 major newspapers, sent 50,000 letters to school principals and had Mitchell spread the word on the road. "The very nature of the product led to the rumor," says a GF spokesman. "Children dream up scary stories. It's part of the excitement."

Though the Rocks will still not reach the West and most of the Southeast until spring, Mitchell has been gulping them himself and enclosing them in family Christmas cards since creating them in the lab in 1956. "We were trying to make a carbonated beverage powder that would taste good," he recalls. "We wanted to put carbon dioxide directly into a solid." That part of the experiment was a flop, but when Mitchell tossed some in his mouth, he saw the future, and it popped. Scientists all over the lab came flocking to sample the stuff. "It became a game—who could swallow the biggest chunk," says Mitchell. "It was a fun afternoon and we wasted a lot of time, but I thought it was a good thing from the start."

General Foods was baffled by the product, however, and spent nearly 18 years trying to decide how to market it. Finally, a Canadian subsidiary began selling it as a novelty, and the home office sat up and took notice. Such is the Rocks' popularity now that a black market has sprung up in some cities, sending prices as high as a dollar a packet. "If you're paying more than 25 cents," says Mitchell, "you're paying too much."

A native Minnesotan, Mitchell graduated from Nebraska Wesleyan University with a degree in physics and chemistry and later taught science in high school. He worked briefly for Eastman Kodak, then joined General Foods in 1941. He retired in 1976 but is still on retainer as a consultant. His wife, Ruth, was once mayor of Lincoln Park, N.J., where they raised their seven now-grown children, all of whom were nurtured on Pop Rocks (and survived to admit it). Dad continues to putter with molecular mysteries. His current quest: a dehydrated melon. "If it were easy," Mitchell mutters over his test tubes, "it would have been done already."