A new study brings a whole new meaning to chestnuts roasting on an open fire. Researchers have found that just 10 minutes with a laptop on your lap brings the temperature of your testicles “above what we consider safe.”

The study found that not only were men’s tender bits heating up quickly, but the subjects have a tendency not to notice. (Oh, and putting a pillow between you and the computer doesn’t make a scrap of difference.)

Why is this a problem? Overheated testes, as you may have heard, have been shown to lower sperm count. While it’s a little early to scream infertility in a crowded room of worried guys, previous research has shown that raising scrotal temperatures by more than one degree Celsius (1.8 degrees Fahrenheit) is enough to damage sperm.

“Millions and millions of men are using laptops now, especially those in the reproductive age range,” said lead researcher Dr. Yefim Sheynkin. “I wouldn’t say that if someone starts to use laptops they will become infertile, [but] the scrotum doesn’t have time to cool down”—which can cause scrotal hyperthermia.

So what can you do? Use a desk.