Anal circumference
An infamous feature of the equally infamous FATAL (Fantasy Adventure To Erotic Lechery) game, the Anal Circumference table enabled players to determine the maximum size of object their character's anus could accomodate without tearing.
Like most aspects of FATAL this is bizarre and terrible not only in concept but in execution, as the numbers the table provides make little sense. What we DO know however is that elderly people have a tighter anus than middle aged ones, since they get a smaller modifier to their roll.