math joke

there is this math joke:

q: "what's purple and commutes?"
a: "an abelian grape."

math geeks explain that abelian sets have the commutative property and grapes are purple, ergo humor.  armed with this "joke", which i didn't completely understand, i set out to entertain the overeducated.  turns out that most math geeks had already heard that one, and the best i could get out of them was a polite chuckle.  my friend richard was from sweden and studying math at mit.  he seemed like a good candidate.

me:      "what's purple and commutes?"
richard: "i don't know."
me:      "an abelian grape."
richard: "um… what's a grape?"
me:      "a small purple fruit."
richard: *laughs* - "that's funny!"

richard was later overhead delivering the punch line to a friend, "an abelian fig! …get it!"