A riddle is "a question or statement intentionally phrased so as to require ingenuity in ascertaining its answer or meaning, typically presented as a game." So, basically, a riddle is a puzzle that is written in a way to make you think about the problem and acquire a solution. Riddles can be found most everywhere, some are even accidental.
One famous riddle is the Riddle of the Sphinx. In Greek mythology, the Sphinx was said to have guarded the entrance to the Greek City of Thebes. Everyone who walked by the Sphinx was asked this riddle. Those who did not answer correctly were devoured or strangled. The exact wording is not known for sure, but according to tales and stories, the riddle is as follows:
Which creature in the morning goes on four feet, at noon on two, and in the evening upon three?
See if you can figure it out!
P.S. You will not get eaten if you get it wrong.