Riddles are essentially word puzzles that use metaphors to describe something very specific. Riddles can also be jokes that are plays on words. These usually take the form of question and answer, for example, "Why was the picture sent to jail?" The answer is that it was framed.
Numerous examples of riddles exist, with some that are simple and many that are not. Complex riddles may be long and may involve obscure cultural references. A simple example is "What gets wetter and wetter the more it dries?" The answer is a towel. This illustrates the fact that riddles often seem to present something that does not make sense upon first hearing it. In that riddle, the answer is dependent on thinking of the towel performing the drying action on a person or thing. This illustrates another characteristic of riddles, and that is of the item taking on the role of the speaker in telling the riddle.
Often riddles challenge what people commonly hold as fact. For example, "What is black when clean and white when dirty?" The answer is a blackboard. This challenges because most people associate an item being black with dirtiness, for example, something covered in soot. Sometimes riddles also employ rhyming schemes, such as rhyming every other phrase.