| reason(noun, information, cause, start, reason) |
| enigma(noun, trouble, informal, challenge, question) |
| ground(noun, cause, start, reason, idea) |
| mystery(noun, trouble, difficulty, reason, informal) |
| wherefore(noun, adverb, information, cause, start) |
| riddle(noun, trouble, challenge, question, secret) |
| what for(verb, expression, phrasal verb) |
| mystification(noun, trouble, challenge, question, secret) |
| problem(noun, trouble, challenge, question, secret) |
| proof(noun, cause, reason, informal, idea) |
| puzzlement(noun, trouble, challenge, question, secret) |
| poser(noun, trouble, difficulty, challenge, question) |
| argument(noun, information, cause, reason, idea) |
| cause(noun, start, reason, base, call) |
| closed book(noun, expression, trouble, challenge, question) |
| explanation(noun, cause, basis, justification, grounds) |
| how(noun, adverb, reason) |
| rationale(noun, cause, reason, idea, case) |
| purpose(noun, cause, consideration, purpose, basis) |
| brain-teaser(noun, trouble, challenge, puzzle) |