(CNN) – Looking at the ultrasounds of 400 knuckles, the researchers saw something striking:

When a knuckle cracked, there was a distinctive and sudden flash in the joint.

They think what’s happening is that when you crack a knuckle, you’re pulling apart two surfaces of the joint, which brings down the pressure in the joint. That negative pressure allows gas that’s dissolved in the fluid in your joints to be liberated, and the bright flash is a gas bubble forming.

“We think that’s why a joint gets more range of motion after you crack it: There’s something about lowering the pressure that allows for more laxity,” Boutin said.