| closed book(expression, question, secret, puzzle, subtlety) |
| conundrum(difficulty, question, secret, puzzle, obscurity) |
| enigma(difficulty, question, secret, puzzle, obscurity) |
| sixty-four-thousand-dollar question(expression, question, secret, puzzle, subtlety) |
| tough nut to crack(expression, idiom, difficulty, question, secret) |
| abstruseness(difficulty, question, secret, puzzle, subtlety) |
| brainteaser(difficulty, question, secret, puzzle, subtlety) |
| braintwister(difficulty, question, secret, puzzle, subtlety) |
| charade(difficulty, question, secret, puzzle, subtlety) |
| chiller(difficulty, question, secret, puzzle, subtlety) |
| cliffhanger(difficulty, question, secret, puzzle, obscurity) |
| crux(difficulty, question, secret, puzzle, obscurity) |
| cryptogram(difficulty, question, secret, puzzle, obscurity) |
| difficulty(question, secret, puzzle, obscurity, subtlety) |
| grabber(difficulty, question, secret, puzzle, perplexity) |
| inscrutability(difficulty, question, secret, puzzle, subtlety) |
| inscrutableness(difficulty, question, secret, puzzle, subtlety) |
| mystery(noun, difficulty, question, secret, puzzle) |
| mystification(question, secret, puzzle, obscurity, subtlety) |
| occult(difficulty, question, secret, puzzle, subtlety) |