floorer (?.)
- 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
