The procedure is simple - Each subject is sent to Africa to hunt elephants. The subsequent elephant-hunting behaviour is then categorized by comparison to the classification rules outlined below. The subject should be assigned to the general job classification that best matches the observed behaviour....There are other variants of this classic.
MATHEMATICIANS hunt elephants by going to Africa, throwing out everything that is not an elephant, and catching one of whatever is left.
EXPERIENCED MATHEMATICIANS will attempt to prove the existence of at least one unique elephant before proceeding to step 1 as a subordinate exercise.
PROFESSORS OF MATHEMATICS will prove the existence of at least one unique elephant and then leave the detection and capture of an actual elephant as an exercise for their graduate students....
And while we're on the subject of Matehmatical jokes, there's Subliminal Nonsense, from a German Mathematics Student currently at Trinity College, Cambridge. Not just humour, but a few small animated .Gifs like the one below, and links to a variety of on-line games.