"Stop doing that!!"... One of Mom's favorite and most frequent expressions (often followed by "You'll put your eye out!") can be provoked in so many ways, especially by cracking your knuckles. That slightly liquid sounding SNAP seems to rub the human pleasure center both ways - much like the stroke of fingernails
on either one's back, or one's blackboard.
Funny how something that provides so much pleasure to one can cause so much annoyance to another... such is the case with cracking your knuckles. The Yubi-Poki virtual knuckle cracker will keep on cracking even after you've run out of knuckles.
holy! They only thing more annoying then someone cracking knuckles is a fake knuckle-cracker!