What double-meaning phrase is put forth as a puzzle to be solved?

A riddle is a statement, question, or phrase having a double or veiled meaning, put forth as a puzzle to be solved. Riddles are of two types: enigmas, which are problems generally expressed in metaphorical or allegorical language that require ingenuity and careful thinking for their solution, and conundra, which are questions relying on their effects on punning in either the question or the answer.

Source: Wikipedia