And for those who don't: Plato, a Greek philosopher, was putatively asked by a student while teaching at the Academy what the definition of a man (human) was. Plato responded that a man is a "featherless biped".

Diogenes, another Greek philosopher and infamous quick-wit, caught wind of this and thought that was the dumbest thing ever, so he gate-crashed one of Plato's lectures and pulled out a chicken which had all of its feathers plucked out and said "Behold, a man!".