When you crack your knuckles, you’re pulling apart the joint, which stretches the capsule of fluid surrounding it, decreasing the pressure within. This causes gasses dissolved in the fluid to release into the space to equalize it. That rush of gas is what creates the pop! sound—like popping the top on a can of soda. It’s unlikely that cracking your knuckles can do the cartilage damage that leads to arthritis, but a longstanding habit can lead to instability in the joint and loss of grip strength down the road.