The knuckles are the joints between the fingers and hand, and these joints contain a lubricant called synovial fluid. When you crack your knuckles, you are pulling apart two bones at the joint, which means the synovial fluid has to fill more space. This decreases the pressure of the fluid, and dissolved gases that are present, such as nitrogen, float out of the area in tiny bubbles. The bursting of these bubbles is the familiar sound we hear when someone "cracks" his or her knuckles. This bubble-bursting is not the same as arthritis, which is when the body's immune system attacks joints. However, constant knuckle-cracking can injure joints and weaken fingers.