According to Dr. Ruderman, when you crack your knuckles, the popping noises or “crepitus” are caused by bursting gas bubbles in the fluid that help lubricate your joints. The bubbles pop when you pull the bones apart, either by stretching the fingers or bending them backward. It releases some endorphins that help reduce pain, but otherwise it’s thought to be a harmless habit that doesn’t signal or cause any type of health problem.