My favorite ancient joke is attributed to Diogenes the Cynic, who lived in a tub and went around with a lantern looking for an honest man, just to mention the most famous stories about him. Here is the story, from Diogenes Laërtius, Lives of the Philosophers (6.40, if you want to look up the Greek):
When Plato had defined Man as a featherless biped, and was admired for it, he [= Diogenes] plucked a chicken and brought it into the lecture-room and said "Here is Plato's Man". After that, 'broad-nailed' was added to the definition.
The Greek actually specifies a male fowl, but 'chicken' sounds funnier in English than 'rooster' and less ambiguous than 'cock'. I often think of Diogenes when passing the poultry bins at the supermarket.
One of Jonah Goldberg's entries in NRO's On the Corner today reminds me of another anecdote about Diogenes (Diogenes Laërtius 6.35):
Posted by Dr. Weevil at March 08, 2002 10:00 PM
He used to say that most people are so nearly mad that a finger makes all the difference. For, if you walk around with your middle finger extended, someone will think you mad, but, if it's the little finger, he will not think so.