| enigma(noun, trouble, challenge, question, secret) |
| mystery(noun, trouble, difficulty, challenge, question) |
| riddle(noun, trouble, challenge, question, secret) |
| conundrum(noun, trouble, challenge, question, secret) |
| secret(noun, trouble, challenge, question, puzzle) |
| brain-teaser(noun, trouble, challenge, puzzle, mystery) |
| mystification(noun, trouble, challenge, question, secret) |
| problem(noun, trouble, challenge, question, secret) |
| puzzlement(noun, trouble, challenge, question, secret) |
| poser(noun, trouble, challenge, question, secret) |
| puzzle(noun, trouble, challenge, question, secret) |
| whodunit(noun, question, secret, puzzle, subtlety) |
| why(noun, trouble, difficulty, challenge, question) |
| arcanum(noun, mystery) |
| mystery story(noun, mystery) |
| crux(noun, trouble, difficulty, challenge, question) |
| sixty-four-thousand-dollar question(expression, question, secret, puzzle, subtlety) |
| tough nut to crack(expression, idiom, question, secret, puzzle) |
| abstruseness(difficulty, question, secret, puzzle, subtlety) |
| brainteaser(difficulty, question, secret, puzzle, subtlety) |