| riddle(noun, difficulty, question, secret, puzzle) |
| puzzle(noun, difficulty, question, secret, obscurity) |
| charade(noun, question, secret, puzzle, subtlety) |
| enigma(noun, difficulty, question, secret, puzzle) |
| problem(noun, difficulty, question, secret, puzzle) |
| anagram |
| closed book(expression, question, secret, puzzle, subtlety) |
| logogriph |
| mystery(noun, difficulty, question, secret, puzzle) |
| sign |
| 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) |
| badge |
| brain-teaser |
| brainteaser(difficulty, question, secret, puzzle, subtlety) |
| braintwister(difficulty, question, secret, puzzle, subtlety) |
| chiller(difficulty, question, secret, puzzle, subtlety) |
| cliffhanger(difficulty, question, secret, puzzle, obscurity) |
| crux(difficulty, question, secret, puzzle, obscurity) |