The venerable master Tank Yuu was walking with his student, Ou Pi. Hoping to prompt the master into a discussion, Ou Pi said:
"Master, I now understand what the koans are about."
After hearing Ou Pi's explanation, Tank Yuu turned to him and with a blank face said:
"Knock, knock."
"Who's there?" - asked Ou Pi.
...
...
...
At that moment, Ou Pi became enlightened.