| closed book(expression, difficulty, question, secret, puzzle) |
| cryptogram(difficulty, question, secret, puzzle, knot) |
| enigma(noun, difficulty, question, problem, secret) |
| mystification(question, secret, puzzle, obscurity, knot) |
| perplexity(noun, difficulty, question, secret, puzzle) |
| problem(difficulty, question, secret, puzzle, knot) |
| puzzlement(question, secret, puzzle, obscurity, knot) |
| sixty-four-thousand-dollar question(expression, difficulty, question, secret, puzzle) |
| sphinx(difficulty, question, secret, puzzle, knot) |
| stickler(difficulty, question, secret, puzzle, knot) |
| stumper(difficulty, question, secret, puzzle, knot) |
| teaser(difficulty, question, secret, puzzle, knot) |
| twister(difficulty, question, secret, puzzle, knot) |
| abstruseness(difficulty, question, secret, puzzle, perplexity) |
| adamant(adjective, mean, hard, severe, harsh) |
| brainteaser(difficulty, question, secret, puzzle, perplexity) |
| braintwister(difficulty, question, secret, puzzle, perplexity) |
| bullhead(adjective, mean, hard, tough, harsh) |
| charade(difficulty, question, secret, puzzle, obscurity) |
| chiller(difficulty, question, secret, puzzle, obscurity) |