Related Clues:

• Sieve
• Difficult problem
• Riddle or puzzle
• Difficult puzzle
• Sticky problem
• Puzzling question
• Certain brainteaser
• It's puzzling
• "When is a door not a door?," e.g.
• ''Cheese'' sayer
• Confusing question
• One in a pic
• Challenging puzzle
• Tough riddle
• It's a mystery