Related Clues:

• It doesn't have an obvious answer
• Inscrutable sort
• The Sphinx had a famous one
• Puzzle or fashion model
• Difficult question
• Confusing query
• "Hmm" provoker
• Sphinx's poser
• It's no no-brainer
• Vexing question
• Appropriately named monthly of the National Puzzlers' League, with 'The'
• Perplexing problem
• WWII encryption machine
• Closed book
• Wannabe