In the year 661, Wonhyo set out on a journey with his friend to learn more about Buddhism in China, which was the regional center of the religion at the time. On a dark night, a rain storm forced the two traveling Buddhist monks to take shelter in a cave. While sleeping there, Wonhyo woke up in the middle of the night as he became very thirsty. He fumbled about in the dark, trying to find something to drink. On the ground, his hand happened to touch a bowl-like vessel filled with water. He thought it was a piece of broken pottery containing water. He picked it up and gulped down the water in it. The water was so cool and refreshing that he fully quenched his thirst and slept well.

The following morning, the monk discovered that skulls and bones were scattered around in the cave, which, in fact, turned out to be an old tomb. He was shocked. But it was even more disturbing to learn that the vessel he had drunk from the previous night was not a bowl, but a human skull. He further realized that he had actually been drinking dirty liquid contained in the rotten skull, instead of fresh water in a bowl. The mere sight of the scene was quite disgusting. He was so repulsed by what he had drunk that he fell on his knees and began to throw up.

Suddenly, he stopped vomiting in startled amazement. He wondered why the sweet and clean water he had drunk with relish in the night became so disgusting the next morning. He realized that it was his mind, not the water itself that determined the difference between truth and reality. He was amazed by how the mind could change perceptions so easily and he realized that truth is created by the mind. That was the very moment when he experienced enlightenment.