Related Clues:

• Sieve
• Puzzling question
• Certain brainteaser
• "When is a door not a door?," e.g.
• It doesn't have an obvious answer
• The Sphinx had a famous one
• Sphinx's poser
• Vexing question
• 'You break it by saying its name,' e.g.
• The large sieve is a puzzle
• Difficult problem or coarse sieve
• Brain-teaser
• Question to test ingenuity, something puzzling
• Kind of puzzle or coarse sieve
• Puzzle or strainer