Synonyms to mind boggler

Chinese puzzle, baffling problem, brain twister, crossword puzzle, crux, enigma, enigmatic question, floorer, jigsaw puzzle, knot, knotty point, mystery, nut to crack, perplexed question, perplexity, poser, problem, puzzle, puzzlement, puzzler, question, question mark, sixty-four dollar question, sticker, stumper, tough proposition, vexed question, why