Related Clues:

• Tough one
• "Cheese" sayer
• Hallucinogen
• Tough problem
• It's not easy
• Mental challenge
• Puzzling problem
• Show-off
• Difficult problem
• Sticky problem
• Puzzling question
• "If three hens lay three eggs in three days ...," e.g.
• ''Cheese'' sayer
• Confusing question
• One in a pic