Chinese puzzle, argument, baffling problem, brain twister, conundrum, crossword puzzle, crux, enigma, enigmatic question, excuse, explanation, floorer, for which, for why, from what cause, ground, how come, jigsaw puzzle, knot, knotty point, mind-boggler, mystery, mystification, nut to crack, on what account, perplexed question, perplexity, poser, pourquoi, pretense, pretext, problem, proof, puzzle, puzzlement, puzzler, question, question mark, rational ground, rationale, reason, reason for, reason why, riddle, sixty-four dollar question, stated cause, sticker, stumper, the big idea, the idea, the whatfor, the wherefore, the why, tough proposition, underlying reason, vexed question, what for, wherefore, whyever, whyfor