In most cases, if your knees crack but don’t hurt, then the noise is just crepitus—gas bubbles bursting, says Tony Gentilcore, C.S.C.S., cofounder of Cressey Sports Performance in Massachusetts.

Sure, crepitus sounds scary, but those gases—oxygen, nitrogen, and carbon dioxide—are just part of synovial fluid, a natural joint lubricant. So relax, you’re fine.

Another harmless knee noisemaker: Your tendons may be clicking back into place after shifting slightly out of their original position.

 What isn’t normal is popping accompanied by pain.

“Some guys push their knees too far forward while squatting, and that puts a lot of undue stress on their joints,” Gentilcore says.

Over time, this causes cartilage to wear down and lose its smoothness, resulting in an audible noise when your bones grind against each other as you bend.

Worst case? You’ll tear your meniscus.

If your knees hurt, it’s probably best to have them checked out by a doctor. You may be told to take a few weeks off from working out to let any swelling die down.