Nail biting, knuckle cracking, pulling out of eyebrows, minor and serious cutting, chewing on hair....all are tension relievers. I gave my kids worry beads to fiddle with and those little balls that you can keep in pockets and roll around. I have no real knowledge but can't imagine that cracking knuckles is a good thing. Often arthritis is caused by not only the inevitabe aging but repetitive activity. (Computer-related hand issues, for example.