A friend of mine, in view of the joke above and the (more famous?) one about Zorn's Lemon, decided to attempt to classify all mathematical fruit jokes. I believe he found only one other; unfortunately it depends on more specialized knowledge than the other two. "What's green and determined up to isomorphism by its first Chern class? A lime bundle."<p>Curiously, I think I have never before heard the abelian grape joke told with a green rather than purple grape.