Once there was a monk who was an expert on the Diamond Sutra, and as books were very valuable in his day, he carried the only copy in his part of the world on his back. He was widely sought after for his readings and insight into the Diamond Sutra, and very successful at propounding its profundities to not only monks and masters but to the lay people as well.
Thus the people of that region came to know of the Diamond Sutra, and as the monk was traveling on a mountain road, he came upon an old woman selling tea and cakes. The hungry monk would have loved to refresh himself, but alas, he had no money. He told the old woman, "I have upon my back a treasure beyond knowing -- the Diamond Sutra. If you will give me some tea and cakes, I will tell you of this great treasure of knowledge."
The old woman knew something of the Diamond Sutra herself, and proposed her own bargain. She said, "Oh learned monk, if you will answer a simple question, I will give you tea and cakes." To this the monk readily agreed. The woman then said, "When you eat these cakes, are you eating with the mind of the past, the mind of the present or the mind of the future?"
No answer occurred to the monk, so he took the pack from his back and got out the text of the Diamond Sutra, hoping he could find the answer. As he studied and pondered, the day grew late and the old woman packed up her things to go home for the day.
"You are a foolish monk indeed," said the old woman as she left the hungry monk in his quandary. "You eat the tea and cakes with your mouth."