The answer to the riddle, "What demands an answer but asks no question?" is a telephone or a doorbell. The riddle is making a pun on the word answer. Instead of a spoken reply, the word is used to mean an action in response to some event.
A riddle is a puzzling question that requires a thoughtful or witty answer. Riddles often consist of a fanciful description or a definition of something to be guessed. Riddles are usually deliberately enigmatic or ambiguous. Riddles are a form of guessing game that have been a part of the folklore of most cultures since ancient times.