| enigma(noun, difficulty, question, secret, puzzle) |
| secret(question, problem, puzzle, obscurity, knot) |
| dragon |
| closed book(expression, difficulty, question, secret, puzzle) |
| chimera |
| hydra |
| riddle(noun, difficulty, question, secret, puzzle) |
| stumper(difficulty, question, secret, puzzle, knot) |
| teaser(difficulty, question, secret, puzzle, knot) |
| tough nut to crack(expression, idiom, question, secret, puzzle) |
| chiller(difficulty, question, secret, puzzle, obscurity) |
| cliffhanger(difficulty, question, secret, puzzle, knot) |
| crux(difficulty, question, secret, puzzle, knot) |
| cryptogram(difficulty, question, secret, puzzle, knot) |
| difficulty(question, secret, puzzle, obscurity, subtlety) |
| grabber(difficulty, question, secret, puzzle, knot) |
| griffin |
| griffon |
| inscrutability(difficulty, question, secret, puzzle, perplexity) |
| inscrutableness(difficulty, question, secret, puzzle, perplexity) |