| riddle(noun, trouble, show, difficulty, challenge) |
| mystery(noun, trouble, show, dispute, difficulty) |
| conundrum(noun, trouble, show, difficulty, difficult) |
| puzzle(noun, trouble, show, dispute, difficulty) |
| secret(noun, trouble, challenge, question, puzzle) |
| problem(noun, trouble, difficulty, issue, challenge) |
| brain-teaser(noun, trouble, difficult, issue, challenge) |
| closed book(noun, expression, idiom, trouble, challenge) |
| paradox(noun, trouble, challenge, question, problem) |
| question(noun, trouble, challenge, doubt, problem) |
| puzzlement(noun, trouble, challenge, question, secret) |
| poser(noun, trouble, difficulty, challenge, question) |
| puzzler(noun, show, difficulty, difficult, issue) |
| arcanum(noun, question, secret, dilemma, mystery) |
| mystification(noun, trouble, challenge, question, secret) |
| perplexity(noun, show, difficulty, question, secret) |
| stumper(noun, trouble, difficulty, difficult, issue) |
| why(noun, trouble, informal, challenge, question) |
| dilemma(noun, trouble, difficult, issue, challenge) |
| crux(noun, trouble, challenge, question, secret) |