You can spin them on your nose, chin, finger or tongue. Originally designed to help calm children with attention-deficit hyperactivity disorder or autism, it then swept the world as a toy that everyone could play with.

Retail sales have undoubtedly slowed recently, said Mark Austin of ToyWorld, a trade publication — good news for the schools that have banned the toys as too distracting for pupils.

But the spinner has created a new "fidget" category of playthings. And the global toy industry has learned lessons from its surprising success.

The fad started in the U.S. in February. By May, all 20 of the top-selling toys on Amazon, an online retailer, were either fidget spinners or fidget cubes, a close relation.

There have been many such crazes, but none that spread as fast. Frédérique Tutt, an analyst of the global toy market for NPD, a data company, said the spinner took just three weeks to cross the Atlantic and go global.

No one knows exactly how many have been sold, with estimates from 19 million to more than 50 million. Big toy retailers, the usual arbiters of what sells, were initially caught flat-footed. Fidget spinners were a plaything that children themselves discovered and shared on social media, particularly on YouTube and Instagram.

Andrew Moulsher, managing director of Peterkin, a firm that imports toys into Britain, called it a "watershed moment" for the business. Big retailers usually plan their inventory as much as 18 months ahead of peak seasons such as summer or Christmas; schedules are often tied to toy-filled films such as the "Star Wars" and "Cars" franchises.

Developing and manufacturing a toy can take even longer than inventory planning — up to three years.

Now there is pressure to spot new fads and bring products to market far more quickly and pick up queues from social media.

The spinner's successor may be the roller, an oblong object weighted at either end. Moulsher started importing Japanese Mokuru rollers into Britain in July and has sold about 40,000. Learning from the fidget fad, he hopes the new school term and a smart social-media strategy will see sales rocket.

Teachers, be warned.

ECONOMIST