| secret(noun, question, puzzle, perplexity, obscurity) |
| sixty-four-thousand-dollar question(noun, expression, question, secret, puzzle) |
| tough nut to crack(noun, expression, idiom, question, secret) |
| puzzlement(noun, question, secret, puzzle, obscurity) |
| abstruseness(noun, question, secret, puzzle, subtlety) |
| bewitchment(perplexity, abstruseness, inscrutableness) |
| brain-teaser(noun, question, puzzle, riddle) |
| braintwister(noun, question, secret, puzzle, subtlety) |
| case(noun, puzzle, riddle, enigma) |
| charade(noun, question, secret, puzzle, subtlety) |
| chiller(noun, question, secret, puzzle, subtlety) |
| cliffhanger(noun, difficulty, question, secret, puzzle) |
| confusing(noun, question, puzzle, riddle) |
| crux(noun, difficulty, question, secret, puzzle) |
| difficult problem(noun, question, puzzle, riddle) |
| difficult question(noun, question, puzzle, riddle) |
| difficulty(noun, question, secret, puzzle, subtlety) |
| dilemma(noun, question, puzzle, mystery, conundrum) |
| grabber(noun, difficulty, challenge, question, secret) |
| inscrutability(noun, question, secret, puzzle, subtlety) |