what's the average airspeed velocity of an unladen swallow

Later, at the Bridge of Death, the Bridgekeeper asks Arthur his third question. "What is the airspeed velocity of an unladen swallow?" When Arthur asks the old man whether he means an African or European swallow, the Bridgekeeper is unable to answer, and is thus launched into The Gorge of Eternal Peril. When Sir Bedevere asks how Arthur knows so much about swallows, he replies, "You have to know these things when you're king."

This is a popular line from a Monte Pyton film, Monty Python and the Holy Grail, where the correct answer to the answer is yet another question "African or European", which tricks the Bridgekeeper into a wrong answer of "I don't know that".