In the ‘90s, slime was the gooey green stuff that got dumped on Nickelodeon game show contestants. Now, it’s a homemade substance that’s somewhere between Silly Putty and Play-Doh. It comes in pretty much every color under the rainbow. And it’s turned into an obsession for kids anywhere between pre-K and high school.

Our doctors and pharmacists are excited that this trend is sparking an early interest in science. But they also want to urge you and your kids to be careful when it comes to slime.

“Most slime recipes out there call for Borax, a strong alkaline substance like lye,” said Mark Johnston, program coordinator of the Regions Hospital Burn Center. “If used improperly or carelessly, it can cause chemical burns.”

According to Johnston, close parent supervision is key to making sure your child stays safe. Here’s what he recommends:

1. Make sure you know what your kids are mixing into their slime.

The amount of Borax that slime recipes call for is not enough to get absorbed through the skin of most people. That said, it’s important to recognize that what a recipe calls for is not always the amount that gets added. This is especially true when it comes to kids. They often have a hard time with the “less is more” concept. And, they can get carried away with dumping in substances not called for in the recipe. While this kind of creativity is generally a good thing, it can be dangerous when it comes to chemicals. (And it’s why you as a parent should also be careful to not mix household cleaners together.)

By staying close during slime production, you can keep an eye on what gets mixed in. You can make sure your kids are diluting the Borax with the proper amount of water. And ultimately, you can help prevent unsafe choices.

2. Limit the amount of time your kids are playing with slime.

When Borax irritates skin, it’s usually pretty mild. However, severe burns from Borax are possible. These are caused by repeated and prolonged contact with it. This is also the case with other chemical burns. The longer a person is exposed, the more severe a burn will be. Be sure to keep tabs on how your child’s skin is reacting to slime, and discontinue use at the first sign of irritation.

If a burn does appear, don’t soak your child’s skin in water. That can make it worse. Instead, thoroughly rinse the area with running water and head to urgent care or an emergency room with pediatric trauma specialists if it’s serious.

3. Be watchful of how your child is playing with slime.

A person’s eyes, nose and throat are more sensitive than the rest of their skin. This means serious irritation can occur quicker in these spots. Enforce a “no touching your face!” rule when your child is playing with slime, and make sure they wash their hands when they are done. It’s also very important your child doesn’t put slime in their mouth. If slime that contains Borax is ingested, call Poison Control right away at 800-222-1222.

If you already know your child has sensitive skin, avoid making slime with Borax altogether. A pinch of baking soda combined gradually with contact lens solution can serve as a substitute. Although sodium borate (the active ingredient of Borax) exists in contact lens solution, too, the amount is minimal and it is already diluted. There are several other Borax-free recipes out there, too.