YOU can spin them on your nose, chin, finger or tongue. Some include LED lights; others resemble a ship’s wheel, or even a skull and crossbones. The fidget spinner has three paddle-shaped blades attached to a central, weighted disc containing ball bearings. Flick a blade and it spins—for as long as 12 minutes, if it’s an advanced model from Japan. Originally designed to help calm children with attention-deficit hyperactivity disorder or autism, it swept the world earlier this year as a toy that everyone could play with.

Retail sales have undoubtedly slowed recently, says 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 America 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—who can forget the great loom-band mania of 2014?—but none that spread as fast. Frédérique Tutt, an analyst of the global toy market for NPD, a data company, says the spinner took just three weeks to cross the Atlantic and go global. No one knows exactly how many have been sold but NPD estimates that at least 19m were sold in the 12 rich-world countries that it tracks (including America and the biggest European markets) during the first six months of this year. Others put the figure at over 50m.

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. No person or firm had a patent on spinners, so with no licensing fees to pay, anyone could make them. They are produced in huge quantities in China, often by firms that previously manufactured smartphone accessories. Others were made using 3D printing. That has been a boon for small shops, which have been able to stock these unbranded goods from wherever they can find them.