Finally, Nan-in stopped talking and offered to serve the professor tea. He brought out his teapot and two empty cups. He filled the professor's cup, and then continued to pour. As he continued, the teacup overflowed.
The professor watched the overflow until he could no longer restrain himself. "Enough! No more can go into the cup!"
"Like this cup," Nan-in said, "you are full of your own opinions and speculations. How can you begin to taste my tea if you do not first empty your cup?"
An amalgam of different stories of the Zen master and the learned man with the tea cup. Stolen from bits of the internet. Some of it's quoted from Bruce lee?