At one time, Einstein traveled all over the United States giving lectures. He traveled by car and soon became quite friendly with the driver.
　　 The driver listened carefully to Einstein’s lecture, which the great scientist gave again and again. One day, he told Einstein that he knew the lecture so well that he was sure he could give it himself. Einstein smiled and said, “Why don’t you give the lecture for me next time?” The driver agreed.
　　 That evening the two of them went along to the lecture hall. Nobody there had seen Einstein before. As the driver took his place on the stage, everybody clapped. Then he began the lecture. Sure enough, he did not make a single mistake. It was a great success, and when it was over, people clapped and clapped. Then he started to leave, shaking hands with everybody, while Einstein followed quietly a few steps behind.
　　 Just before they got to the door, a man stopped them and asked the driver a very difficult question. The driver listened carefully. Of course, he did not understand a thing, but he nodded his head as if he did. When the man stopped talking, the driver said that he thought the question was very interesting but really quite simple. In fact, in order to show how simple it was, he would ask his driver to answer it.