A Pop It is a fidget toy where users repeatedly pop dimples in and out - similar to bubble wrap.
It's an endless cycle because, after you've popped out all of one side, you just flip it over and start popping again.
The Pop It has become really popular on TikTok, with the hashtag #PopIt getting more than 2.5 billion views.
Toy stores in the UK have taken note of the growing trend by buying millions of Pop Its for their shops.
Hayley Dolman, senior buyer at The Works, told Newsround: "We've seen phenomenal sales, selling thousands every week."
"We knew this would be a real trend even though our stores were closed at the start or the year. In early February we air freighted stock in."
What is a sensory toy?
Fidget toys like Pop Its have been used by health care professionals as a way of helping children who have anxiety or sensory difficulties. This is where a child finds it difficult to respond to information from their senses. So, they may react badly to anything that triggers their senses including light, sound, touch, taste or smell.
Dr Will Shield is a child psychologist from the University of Exeter and he told Newsround that sensory toys like this have been around for generations: "Sensory toys have been used for years, as far back as the 1970s.
"Psychologists in the Netherlands created sensory rooms to help children with disabilities, where they were interested in lights, sounds and objects to play with and touch. This helped to calm and relax people during therapy sessions.
"Sensory toys activate one or more of our senses, for example our sight, hearing or our sense of touch.
"So when we are feeling a big emotion - like being scared or worried - then playing with something that looks, feels or sounds different can help take our attention away from that big emotion.
"Sensory toys can also help us with concentration and attention, just like how adults play with a pen when they're trying to listen to something."
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