What causes the noise when you crack a joint? Is joint cracking harmful?
The exact mechanism is unclear. Here are some possible causes:
There are no known bad effects of joint cracking [1, 4].
And no good effects either:
Is joint-cracking harmful? No. Donald Unger was told by his mother that he'd get arthritis if he cracked his knuckles so he cracked his left knuckles every day for 60 years but never his right knuckles. He had no arthritis or any other problems in either hand and got a publication (D. L. Unger, "Does knuckle cracking lead to arthritis of the fingers?", Arthritis and Rheumatism 41(5):949–950, 1998 – online version for those with appropriate access) and an Ig Nobel Prize (2009) for his troubles.
OK, so one guy's knuckles prove nothing but who can resist the opportunity to cite an Ig Nobel winner?