Possible Clues:

• Zen paradox
• Zen riddle
• Zen question
• Upsetting in Louisiana, Oklahoma? That's a riddle you won't work out!
• Zen master's paradox
• Zen master's question
• Zen puzzle
• "What is the sound of one hand clapping?" e.g
• Knock out a new riddle
• Zen paradox; OK, no (anag.)