Related Clues:

• Nearly unsolvable
• Word with boil or line
• "A ___ Day's Night"
• Difficult to solve
• Verifiable, as facts
• Baffling
• Opposite of soft
• Like some cider
• Rocky
• Complicated
• Merciless
• Type of candy or cider
• Challenging
• Not a walk in the park
• Not a breeze