Squishy toys recently became one of the most popular collectibles among kids. But now, experts are warning about a dangerous social media trend involving the toy that can lead to serious burns.

In the new trend, kids put the squishy toys in a microwave to watch them expand. However, when toys like these are heated, the material inside can reach very high temperatures. If the toys break or explode, the contents inside can shoot out and cause burns to the skin, face or eyes.

“Many children watch these videos online and think they’re harmless. What they don’t realize is that these burns can leave permanent scars and could require surgery or even skin grafts. No online challenge is worth a trip to the hospital,” says Alejandro Garcia, M.D., director of the Johns Hopkins Children’s Center Pediatric Burn Program.

Garcia also warns about leaving the toys in cars. The jelly-like substance inside the toys can reach high temperatures just from being left in a car on a hot day.

“If a burn does occur, cool the area with cool water for about 20 minutes. Do not use ice, butter or other home remedies. Then, seek medical attention,” says Garcia. “Parents should also talk to kids about the danger of these trends. If an online video tells them to heat, set fire to, or modify a toy, they should simply not do it.”