Chinese puzzle
baffling problem
brain twister
campaigner
crossword puzzle
crux
electioneer
enigma
enigmatic question
floorer
jigsaw puzzle
knot
knotty point
mind-boggler
mystery
nut to crack
perplexed question
perplexity
poser
problem
puzzle
puzzlement
puzzler
question
question mark
sixty-four dollar question
sticker
stump orator
stump speaker
tough proposition
vexed question
whistle-stopper
why
bumper
dumper
jumper
lumper
mumper
thumper