Closed book
Else clues• ___ Variations: Elgar opus
• Hmm? inducer
• Hmm provoker
• Hmmmmmm provokeer
• I'm a walking, talking ___: Larry David
• Return to Innocence artist
• Return to Innocence one-man band
• 1982 Martin Sheen film
• Appropriately named monthly of the National Puzzlers' League, with The
• Axis enciphering machine
• Baffler
• Baffling problem
• Brain twister
• Brainteaser
• Bumfuzzlement producer
• Challenging riddle
• Churchill said one enclosed a mystery
• Complete baffler
• Conundrum
• Cracked object of WWII
• Cryptic mystery
• Elgar's ___ Variations
• Encryption machine of yore
• Gamine playing with puzzle (6)
• German cipher machine
• Head-scratcher
• Inscrutable person
• Inscrutable sort
• It's a puzzlement
• It's hard to fathom
• It's hard to figure out
• It's hard to understand
• It's inexplicable
• It's mind-boggling
• It's no no-brainer
• It's not clear
• It's puzzling
• It's something of a mystery
• Knot
• Mind-boggler
• Mona Lisa's smile, for one
• Mystery
• National Puzzlers' League publication, with The
• Perplexer
• Perplexity
• Poser
• Problem
• Puzzle
• Puzzle or code
• Puzzle or riddle
• Puzzlement
• Puzzling person
• Puzzling situation
• Puzzling sort
• Puzzling state
• Puzzling thing
• Real poser
• Riddle
• Riddle or puzzle
• Riddle relative
• Something hard to grasp
• Something inexplicable
• Something inscrutable
• Something of a mystery
• Something puzzling
• Sphinx
• Stumper
• They had a hit with Return to Innocence
• Tough nut to crack
• Tough one to figure out
• Tough puzzle
• Toughie
• World War II encoding machine
• WWII cipher machine
• WWII code machine
• WWII coding machine
• WWII encryption machine
• Something that baffles understanding and cannot be explained
• A difficult problem