Pour one glass of water into another. Now ask yourself: how can you tell which water came from the first glass, and which from the second? As you think about this, you're struck by a bird. The bird survivesbut never forgets.

The koan: "A Letter to a Dying Man"

Bassui wrote the following letter to one of his disciples who was about to die:

"The essence of your mind is not born, so it will never die. It is not an existence, which is perishable. It is not an emptiness, which is a mere void. It has neither color nor form. It enjoys no pleasures and suffers no pains.

"I know you are very ill. Like a good Zen student, you are facing that sickness squarely. You may not know exactly who is suffering, but question yourself: What is the essence of this mind? Think only of this. You will need no more. Covet nothing. Your end which is endless is as a snowflake dissolving in the pure air."

The enlightenment: With that, Bassui set down his pen and admired his work. "Yes, that will do," he said to himself. "That will do very nicely."

He folded the letter evenly, put it in an envelope, and addressed it with simple yet beautiful calligraphic strokes. He stuck the letter in his coat and, whistling a pleasant tune, went out for a leisurely lunch before ambling over to his dying friend's house. When he got there, he found that his friend had just died.

"What do you have there?" the man's grieving wife asked Bassui.

"Oh, uh... pornography," Bassui said. Then he ran home and published his letter in a book instead.

Read more here:
Zen Koans Explained: "A Letter to a Dying Man"