RE: knuckle cracking and arthritis
I actually asked a chiropractor this question a few weeks ago. He told me about a study done in the 1990s where elderly people were separated into three groups--people who've never cracked their knuckles, people who crack their knuckles a couple of times a day, and people who crack their knuckles more than twice a day. The group that suffered arthritis the most (most cases of arthritis) were the people who NEVER crack their knuckles. The group with the least arthritis was the group who cracks once or twice a day.
He said that what you're really doing is stretching the joint and that it can be good for you. Hey, that's basically what chiropractors do right--crack your bones? Only they do it to your back and neck.
He also said that cracking your joints releases endorphins and that is why it feels good and also why it is addictive.
Do I crack my knuckles? Yup, since sixth grade. It annoys my wife to no end.