Chinese puzzle
baffling problem
brain twister
crossword puzzle
crux
enigma
enigmatic question
floorer
jigsaw puzzle
knot
knotty point
mind-boggler
mystery
nut to crack
perplexed question
perplexity
poser
problem
puzzle
puzzlement
question
question mark
sixty-four dollar question
sticker
stumper
tough proposition
vexed question
why
acrostic
sudoku
word square
6-minute
Mozart
archived
blackleg
bronchopulmonary
costumer
freeware
pie-shaped
reportable
sceneries
slipslop
smiting
sweet-stuff
guzzler