"Which came first, the phoenix or the flame?"
-- The bronze eagle door knocker of the Ravenclaw Common Room, when Harry and Luna attempt to enter (DH29).
The Ravenclaw Common Room door knocker is in the form of a bronze eagle. It guards the Ravenclaw Common Room by asking visitors a question, which they must answer correctly in order to gain entry. It has a soft, musical voice and a beak that opens when it speaks. The questions are in the form of puzzles or riddles that require logic and reasoning to answer.
Just before the Battle of Hogwarts, Harry and Luna go to the Ravenclaw Common Room to look at a statue of Rowena Ravenclaw wearing her diadem. The eagle door knocker presents them with a question they must answer in order to gain entry (DH29). "Which came first, the phoenix or the flame?"
Later, Amycus Carrow is unable to gain entry after he failed to answer the door knocker's question correctly. He attempts to force his way through the door, until Professor McGonagall appears and answers the question (DH29). "Where do Vanished objects go?"
Luna Lovegood's house also has a door knocker in the shape of an eagle.