Synonyms to think hard

hammer away at, always trot out, apply the mind, belabor, dig, drudge, dwell on, dwell upon, fag, grind, grub, hammer, hammer at, hammer away, harp on, insist upon, keep doggedly at, labor, moil, peg, peg away, plod, plug, plug along, plug at, plug away, pound away, puzzle, puzzle over, slog, slog away, stew over, thresh over, toil, travail, wade through, work away, agonize over, be at sea, be uncertain, beat about, doubt, feel unsure, flounder, grope, question, thrash about, wonder, wonder whether, Chinese puzzle, Gordian knot, addle, ail, amaze, baffle, bafflement, baffling problem, bamboozle, be the matter, beat, befog, befuddle, beset, bewilder, bewilderment, boggle, bother, brain twister, brain-teaser, 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