difficult / hard, probably impossible

untie; unravel; undo (a fastening); unwrap / dispel (doubts); clear up (a matter)

solve (a problem); answer (a question); unravel (a riddle) / comb; untangle (hair)

riddle; enigma

mystery; puzzle

riddle / puzzle / enigma / hint / tip

be enigmatic

(informal) "mystery meat", specifically the meat-based product found in Nissin Cup Noodles