First there were Pokemon cards and Rubik’s cubes. Then rainbow looms, dabbing and bottle flipping. This spring, the hot new trend on the schoolyard is fidget toys.

So, what are they?

Fidget toys are tiny, pocket-sized toys intended to keep hands busy. A step up from stress balls, they typically feature little buttons, switches and joysticks that move back and forth, or propellers that spin.

That’s pretty much it. That’s all they do.

But the toys are being marketed as stress relievers or to help kids burn up pent-up energy that so often gets them into trouble. Some claim to have been modelled after stress tools for kids with ADHD (attention deficit hyperactivity disorder); others say they’re just for fun.

The most popular of the fidget toys are fidget spinners that are mesmerizing to simply watch, but can also be used for tricks.

The spinners typically feature two or more propellers and a low-friction, ball bearing centre that allow them to spin for a minute or more at a time. While they spin, users transfer them from hand to hand or finger to finger, while tossing them in the air.

Videos on YouTube show viewers how to perform spinner tricks, touting the latest and most expensive versions as those with the best and longest spins.

Plastic spinners are relatively cheap at just a couple of dollars each, but the highest-end versions in metal alloys can run range in price from $15 to $100 each or more.

Despite such high price tags, fidget spinners have become huge sellers in recent weeks, with toy stores struggling to keep them in stock.