There was once a university professor who lived in Japan. He was a professor of philosophy, and he wanted to know more about the philosophy of Zen Buddhism. So he visited a Zen master by the name of Nan-in.
Nan-in welcomed the professor, and the two sat at a table to begin their discussion of Zen. The professor claimed he wanted to learn, but he was already an expert in philosophy. Nothing Nan-in could have said would have sounded particularly ground-breaking to the professor. In the professor’s mind, he already knew everything Nan-in had to say about Zen.
Nan-in offered his guest a cup of tea. He placed a cup before him and began to fill it. The professor watched as the Zen master filled the cup to the brim. Nan-in kept pouring the tea, and the cup began to overflow. And still he poured the tea, which flowed onto the dish that held the teacup and on across the table.