Advice from the mountain

Once a very old king went to see an old hermit who lived in a bird's nest in the top of a tree, "What is the most important Buddhist teaching?" The hermit answered, "Do no evil, do only good. Purify your heart." The king had expected to hear a very long explanation. He protested, "But even a five-year old child can understand that!" "Yes," replied the wise sage, "but even an 80-year-old man cannot do it."