Good riddles and answers can range from relatively easy, like "What kind of room has no doors or windows? A mushroom" to the very difficult, such as the Sphinx's riddle. This famous riddle asks, "What walks on four legs in the morning, two legs in the afternoon, and three legs at night?" The answer, "a person," requires the guesser to imagine a person's life as a day, beginning with the morning (crawling) and ending with night (walking with a cane).
Some of the best riddles are challenging because they use multiple definitions of the same words to create a puzzle for the guesser. For example, one riddle asks for something with a mouth that doesn't talk, a head and a bed. To figure out the answer, the guesser has to think of something other than a person that fits the answer: a river.
Other good riddles revolve around words and letters, such as: "Forward it's heavy, backwards it's not." Here, the answer, "ton," is in the question, since the word "not" is "ton" backwards. A similar riddle is "What occurs once in a minute, twice in a moment, but never in a century?" All of these words measure time, intentionally misdirecting the listener from the answer, the letter M.