| difficulty(noun, trouble, challenge, question, secret) |
| question(noun, trouble, challenge, secret, brain) |
| problem(noun, trouble, challenge, question, secret) |
| dilemma(noun, trouble, challenge, question, secret) |
| quandary(noun, trouble, challenge, question, secret) |
| puzzlement(noun, trouble, challenge, question, secret) |
| mystery(noun, trouble, challenge, question, secret) |
| stumper(noun, trouble, challenge, question, secret) |
| brain-teaser(noun, trouble, challenge, question, secret) |
| deal breaker(noun, challenge, case, matter, knot) |
| riddle(noun, trouble, challenge, question, secret) |
| enigma(noun, trouble, challenge, question, secret) |
| brainteaser(noun, trouble, challenge, question, secret) |
| paradox(noun, brain, mystery, secrecy) |
| physics(noun, issue, challenge, case, matter) |
| poser(noun, brain, riddle, brainteaser) |
| predicament(noun, brain, riddle, brainteaser) |
| teaser(noun, brain, riddle, brainteaser) |
| puzzle(noun, trouble, challenge, question, secret) |
| difficult question(noun, expression, trouble, challenge, question) |