- anagram
- baffle
- bafflement
- baffling problem
- bewilderment
- bother
- brain twister
- charade
- Chinese puzzle
- confoundment
- confusion
- conundrum
- crossword puzzle
- crux
- dark horse
- dilemma
- discomposure
- disconcert
- disconcertedness
- disconcertion
- disconcertment
- disturbance
- embarrassment
- enigma
- enigmatic question
- fix
- floorer
- frontier
- frontiers of knowledge
- hit
- jam
- jigsaw puzzle
- knot
- knotty point
- matter of ignorance
- mind-boggler
- mystery
- mystification
- n
- nonplus
- nut to crack
- perplexed question
- perplexity
- perturbation
- pickle
- plight
- poser
- pother
- predicament
- problem
- puzzle
- puzzlement
- puzzler
- quandary
- question
- question mark
- rebus
- riddle
- scrape
- sealed book
- sixty-four dollar question
- stew
- sticker
- stumper
- terra incognita
- the incalculable
- the strange
- the unfamiliar
- the unknowable
- the unknown
- tough proposition
- unassuredness
- unexplored ground
- unexplored territory
- unknown quantity
- upset
- vexed question
- why
- work
- X
- Z