The toy slime that’s sold in stores and online is supposed to be squishy, gross, and fun. Unfortunately, some of it might also be toxic.

A consumer group in the United Kingdom recently tested 11 popular brands of slime, many of which are also sold in the U.S. Eight of those brands contained unsafe levels of the element boron (B). Boron is the ingredient that makes slime sticky. It’s not harmful in small doses. But large amounts of the element can cause skin irritation, vomiting, diarrhea, and harm to the reproductive system. It’s not something you want in excess on your fingers.

In its report, the consumer group also warned about the safety of homemade slime. Many DIY slime recipes include the chemical borax or contact lens solution, both of which contain boron in varying quantities. Look for boron-free recipes if you want to make slime safely.