Once the monks of the Eastern Hall and the Western Hall were disputing about a cat. Nansen, holding up the cat, said, "Monks, if you can say a word of Zen, I will spare the cat. If you cannot, I will kill it!" No monk could answer. Nansen finally killed the cat.
In the evening, when Joshu came back, Nansen told him of the incident. Joshu took off his sandal, put it on his head, and walked off. Nansen said, "If you had been there, I could have saved the cat!"--- From The Gateless Barrier:
Zen Comments on the Mumonkan