When you “crack” your knuckles, or any other joint, it expands the space between your bones, creating negative pressure that draws synovial fluid into the new gap. This influx of synovial fluid is what causes the popping sound and feeling when you crack a knuckle. Although it gives a pschycological relief from pain . But habitual cracking or popping of knuckles is not a good thing in the long run as it loosens the ligaments and may lead to problems at a later stage of life ( Early Arthritis , Loosening of Grip )