You might love your fancy sponge, but what are you REALLY smearing on your face? Users on one Reddit board got curious and cut their beauty sponges open.

If they aren’t cleaned properly, foundation seeps into the centers of the sponges. And if the problem is really bad, they can start growing mold.

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“That’s a problem,” explains ER Physician Dr. Travis Stork, “Because then you’re applying bacteria and mold to your face.” Plastic Surgeon Dr. Andrew Ordon points out that this can infect pores and lead to breakouts.

Dr. Stork adds that some brands of sponge cost $20 or more, which makes users reluctant to throw them out. Pediatrician Dr. Tanya Altmann agrees and says she loves the cheap disposable sponges from the beauty supply store – but if you’re going to go upscale, wash your sponge often with baby soap and water.

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Dr. Ordon’s takeaway is that the key is clean – clean makeup, clean tools, and clean skin. And never share tools or makeup!