The Gateless Gate/Keichu's Wheel
Getsuan said to his students: "Keichu, the first wheel-maker of China, made two wheels of fifty spokes each. Now, suppose you removed the nave uniting the spokes. What would become of the wheel? And had Keichu done this, could he be called the master wheel-maker?"

 Mumon’s comment: If anyone can answer this question instantly, his eyes will be like a comet and his mind like a flash of lightning.
 When the hubless wheel turns,
 Master or no master can stop it.
 It turns above heaven and below earth,
 South, north, east, and west.