Don't torture yourself trying to reverse-engineer it from pure memory - most riddles rely on a specific wordplay or lateral thinking angle that you either catch or you don't, and forcing it usually just locks your brain up more. Better move is to think about what *category* the riddle fell into: was it a pun, a logic puzzle, something that plays on double meanings? If you can narrow that down, it might jog the actual answer. If you're still stuck after that, just hit up your friend - they'll probably enjoy explaining the trick to you, and honestly half the fun of riddles is sharing that "oh damn, I didn't see that coming" moment anyway.