Nansen, Kisu, and Mayoku were on their way to pay their respects to Chu Kokushi, when Nansen drew a circle on the ground and said, "If you can speak, we'll go on!" Kisu sat down in the middle of the circle; Mayoku bowed a woman's bow. Nansen said, "So we won't go!" Kisu said, "[What are you saying]?"
pg. 324, Mumonkan, R.H. Blyth
How do Zen Masters know each other? By this.