A ‘Zen koan’ is a story, dialogue, question, or statement, which is used in Zen practice to provoke the thought and to test a student’s progress in Zen practice. Zen koans are often witty, bewildering, and always amusing to ponder. To ordinary, non-zen-practicing folks (like me), koans can be read like riddles, to test and entertain one’s mental capacity. It is important to always keep in mind though, that koans are not mere riddle; it is far deeper than that. I like to think that Zen koans is to riddles what igo is to chess. The koan below is taken from the book titled ‘Sands and Pebbles’ (Japanese: 沙石集 shasekishuu), translated from Japanese into English by Nyogen Senzaki and Paul Reps.
Nan-in, a Japanese master during the Meiji era (1868-1912), received a university professor who came to inquire about Zen. Nan-in served tea. He poured his visitor’s cup full, and then kept on pouring. The professor watched the overflow until he no longer could restrain himself. “It is overfull. No more will go in!” “Like this cup,” Nan-in said, “you are full of your own opinions and speculations. How can I show you Zen unless you first empty your cup?”