"It's a story from long, long ago," continued the old man. "Once upon a time, there was a wise and able king who eagerly sought out capable people. He ordered that his minister find the smartest person in the entire realm, so as to appoint him the teacher of the prince. One day he could assist the prince when he ruled.--The City Trilogy, by Chang Hsi-kuo.
"The minister himself was extremely intelligent. He scoured the land, leaving no stone unturned, until he found the three smartest people. The minister thought of every way he could to test the three. But unexpectedly, he was unable to determine which one was smartest. The minister was embarrassed because the king had instructed him to find the single smartest person in the realm. He had to do something to find out who it was.
"After giving the matter much thought, he came up with a solution. He assembled the three people and asked them to look at five jade disks on a table. Then he said, 'Three of these jade disks are blue and two are green. I am going to blindfold you, after which I will place one disk on top of your head. Then I will remove the blindfolds. You will be able to see the disks on top of the others' heads but not the one on top of your own. You cannot speak nor gesture to one another, and anyone who breaks the rules will be put to death on the spot. The first one to guess the color of the disk on his own head will be deemed the smartest person in the land, and will become the prince's teacher. But if you guess incorrectly, you will be executed at once.'
"So saying, he blindfolded the three smart people and placed one jade disk on top of each one's head. Then he removed the blindfolds. The three people looked at one another for some time, but no one said a word. Finally, one of them got it, guessed the color of the disk on top of his head, and became the prince's teacher. Later he himself became minister and enjoyed wealth and a high position his entire life.
"So what color were the disks on top of the people's heads?"