Chinese puzzle, Gordian knot, addle, ail, amaze, apply the mind, baffle, bafflement, baffling problem, bamboozle, be the matter, beat, befog, befuddle, beset, bewilder, bewilderment, boggle, bother, brain-teaser, brain twister, buffalo, cap, complicate matters, concern, confound, confoundment, confuse, confusion, consider, contemplate, conundrum, crossword puzzle, crux, dark horse, daze, decipher, dilemma, discommode, discomposure, disconcert, disconcertedness, disconcertion, disconcertment, distract, distress, disturb, disturbance, dumbfound, embarrassment, enigma, enigmatic question, figure out, fix, flabbergast, floor, floorer, flummox, foil, frontier, frontiers of knowledge, frustrate, fuddle, get, hammer at, hammer away at, harass, inconvenience, irk, jam, jigsaw puzzle, keep in suspense, knot, knotty point, lick, matter of ignorance, maze, mind-boggler, muddle, mull over, muse on, muse over, mystery, mystification, mystify, n, node, nodus, nonplus, nut to crack, oxymoron, paradox, perplex, perplexed question, perplexity, perturb, perturbation, pickle, plague, plight, ponder over, pons asinorum, pose, poser, pother, predicament, problem, put out, put to it, puzzle out, puzzle over, puzzlement, puzzler, quandary, question, question mark, reflect over, riddle, scrape, sealed book, sixty-four dollar question, solve, sort out, stew, stew over, stick, sticker, study, stumble, stump, stumper, stymie, teaser, terra incognita, the incalculable, the strange, the unfamiliar, the unknowable, the unknown, think hard, think through, throw, torment, tough proposition, trouble, unassuredness, unexplored ground, unexplored territory, unknown quantity, unlock, unravel, upset, vex, vexed question, why, work out, worry, x, z