| conundrum(noun, trouble, difficulty, informal, challenge) |
| enigma(noun, trouble, challenge, question, puzzle) |
| riddle(noun, trouble, challenge, question, puzzle) |
| mystery(noun, trouble, challenge, question, puzzle) |
| closed book(noun, expression, trouble, challenge, puzzle) |
| secret(noun, trouble, challenge, question, puzzle) |
| problem(noun, trouble, challenge, question, puzzle) |
| puzzle(noun, trouble, challenge, question, mystery) |
| puzzlement(noun, trouble, challenge, question, puzzle) |
| arcanum(noun, mystery) |
| poser(noun, trouble, challenge, question, puzzle) |
| whodunit(noun, mystery) |
| mystification(noun, trouble, challenge, question, puzzle) |
| paradox(noun, trouble, challenge, question, puzzle) |
| mystery story(noun, mystery) |
| why(noun, trouble, challenge, puzzle) |
| difficult problem(noun, question, puzzle, riddle) |
| dilemma(noun, trouble, challenge, question, puzzle) |
| stumper(noun, question, puzzle, dilemma, complexity) |
| teaser(noun, question, puzzle, dilemma, complexity) |