Nan-in, a Japanese Zen master, received a visit from a university professor who wanted to know about zen. They sat and visited and talked for a while.
Nan-in then took out tea cups and tea and proceeded to pour tea for his guest. He filled his guest's cup and continued to pour as the tea spilled over the sides. The professor, unable to restrain himself, cried : "Stop. The cup is full, no more will go in." Nan-in replied: "Like this cup, you too are full. You area already full of your own judgments, speculations, ideas and opinions. How can you begin to learn until you empty your cup first?"
The lesson for us is that we cannot learn if we believe we already know everything. How often do you think you are open but in reality, your cup is already full?