In November1957, the Chairman of the Chinese Communist Party Mao Zedong les a Chinese delegation to visit the Soviet Union for the second time.

The Soviet Union sent a special plane for Mao Zedong and his party. The Soviet Ambassador to China Uken accompanied Mao in the same questions.

“You are a philosopher and also our old friend,” Mao said to him. “May I test you with a question?”

For a moment Uken felt puzzled and replied: ”Very well, I hope I can pass.”

“Just now we were at the airport. Now we are in the sky and sometime later the plane will land again. How do you explain that philosophically?”

Uken was caught out and said: “Oh dear! I ’ve never thought of this before.”

“Well, so you’ve failed. Let me try to answer it and you will be the judge,” Mao said. “The plane on the ground at the airport is positive. It is negative when flying in the sky. When it lands again it is negative of the negative.”

All present in the plane laughed. Uken also laughed and said: “Good! Good! This answers the question completely.”