Related Clues:

• It's hard to understand
• Riddle relative
• Tough nut to crack
• It's not clear
• Puzzling person
• You're doing one
• Double ___ play
• Puzzling problem
• Difficult problem
• Riddle or puzzle
• Challenging puzzle
• It's a mystery
• Inscrutable sort
• "Hmm" provoker
• It's no no-brainer