Chinese puzzle
baffling problem
brain twister
clincher
conclusive argument
crossword puzzle
crusher
crux
enigma
enigmatic question
finisher
jigsaw puzzle
knockdown argument
knot
knotty point
mind-boggler
mystery
nut to crack
perplexed question
perplexity
poser
problem
puzzle
puzzlement
puzzler
question
question mark
settler
sixty-four dollar question
sockdolager
sticker
stumper
tough proposition
vexed question
why