Oooh! Dead useful this one... a goodly stack of decent riddles (and their answers) so that you can baffle and bamboozle your players to your heart's content.
Just think of the uses. That old wizard who won't tell you anything until you have proven your mental worth by giving him the correct answer. The dragon who just might not gobble you up if you entertain her. That last quiz show question that stands between your character and a heap of money... or some evil supervillain who delights in quizzing superheroes. Or maybe your bard character might like to add a few riddles to his act.
(And being a teacher in real life, they make jolly good lesson starters too!)
Say my name and I'll disappear. What am I? (No, not a DriveThruRPG featured reviewer!)
I shall return and return to this...