Things fall all the time. It’s the gravity thing. But every once in a while something tumbles and someone notices and the world is never the same. Take Newton’s apple. And then again in 1943 when mechanical engineer Richard James noticed a torsion spring fall off a shelf and come to life as it moved over a table and on to the floor. There is no evidence he yelled, “Eureka,” but James did take the spring home and tell his wife Betty, “I think I can make a toy out of this.”

OK, so Slinky didn’t change the world, but James’ sharp eye did give us one nifty little plaything. And one with real staying power. Since becoming the hit of the 1946 American Toy Fair, Slinky sales have totaled more than 250 million — roughly one for every man, woman and child in the United States. And this from a toy that can’t talk, fire rockets or wet itself.

Still for all its simple charm, the Slinky was no overnight success. It took Richard a few years to find steel wire that would coil, uncoil and recoil with the grace of Astaire.

In the meantime Betty scanned the dictionary for a fitting name. She zeroed in on Slinky because it meant “stealthy, sleek and sinuous.” And so with $500 of borrowed money, the couple started making Slinkys. At first their product, with no name recognition, didn’t exactly roll off the shelves. Desperate to cash in on the Christmas rush in late 1945, the Jameses talked a buyer from the Gimbel’s store in Philadelphia into letting them do a demonstration. Fearing the worst, Richard slipped a buck to a friend to make sure at least one Slinky was sold.

But that snowy night Slinky ruled. All 400 toys sold in 90 minutes.

With a big boost from the following year’s toy fair, Slinky sales soared. Richard designed the machinery that could transform 80 feet of wire into two inches of coiled fun. Slinky rolled on, joined in 1950 by the cuter, compact Junior Slinky. But alas, in 1960 the toy story turned tabloid tale. Concluding that being the Slinky King wasn’t enough, Richard James bailed out for Bolivia, joining what Betty describes as a religious cult. She, meanwhile, was left with the company, six kids and a load of debt, largely the result of her husband’s largesse toward his spiritual suitors.

But Betty saved Slinky, selling the Philadelphia factory and moving the operation to the small, western Pennsylvania town of Hollidaysburg. She steered its comeback with co-op advertising and a simple jingle that remains lodged in the brains of Baby Boomers everywhere. (”It’s Slinky, it’s Slinky, for fun it’s a wonderful toy/It’s Slinky, it’s Slinky, it’s fun for a girl and a boy. …”) There have been few other changes. The prototype blue-black Swedish steel was replaced with less expensive, silvery American metal; later a plastic model was added. For safety reasons the Slinky’s ends were crimped in 1973. Clever people have found other uses for James’ toy, most notably soldiers in Vietnam, who found it made a great radio antenna when strung over tree branches. But today’s Slinky is not much different from the original. It’s still made on Richard James’ machines. And at $2, it costs only twice what it did 50 years ago.

Betty James, now 88, still runs the company. And business is as good as ever, thanks in part to a strong supporting role for the Slinky Dog in last year’s Toy Story. This was not, however, Slinky’s big-screen premiere; the toy claimed a cameo in John Waters’ Hairspray and an even more memorable role in Jim Carey’s Ace Ventura: When Nature Calls. It is so deep in our culture now that not only does Neiman-Marcus sell an $80 gold version but also, according to a recent survey, 90 percent of Americans know what a Slinky is. Which makes you wonder: What gives with the other 10 percent?