| mystery(noun, difficulty, challenge, doubt, question) |
| mystification(question, secret, puzzle, obscurity, knot) |
| nitpicker |
| painstaking(adjective, thorough, meticulous) |
| perplexity(noun, difficulty, question, secret, puzzle) |
| persevering(adjective, thorough, meticulous, painstaking) |
| persnickety(adjective, careful, thorough, meticulous, fussy) |
| picky(adjective, careful, thorough, meticulous, fussy) |
| poser(noun, difficulty, challenge, question, secret) |
| punctual(adjective, thorough, meticulous, painstaking) |
| puzzle(noun, difficulty, challenge, question, problem) |
| puzzlement(question, secret, puzzle, obscurity, knot) |
| sedulous(adjective, thorough, meticulous, painstaking) |
| sixty-four-thousand-dollar question(expression, difficulty, question, secret, puzzle) |
| strenuous(adjective, thorough, meticulous, painstaking) |
| stumper(noun, difficulty, challenge, question, problem) |
| teaser(noun, difficulty, challenge, question, problem) |
| thoroughgoing(adjective, thorough, meticulous, painstaking) |
| tough nut to crack(expression, idiom, question, secret, puzzle) |
| twister(noun, difficulty, challenge, question, problem) |