A university professor went to visit a famous Zen master. The professor had studied the concepts of Zen for a long time and was eager to learn more.
While the master quietly served tea, the professor talked about his vast knowledge of Zen. The master poured the visitor's cup to the brim, and then kept pouring.
The professor watched the overflowing cup until he could no longer restrain himself.
"It's overfull! No more will go in!" the professor blurted.
"You are like this cup," the master replied, "You are already full of what you know. How can I show you Zen unless you first empty your cup?"