Not long after his arrival in Princeton, Albert Einstein was invited, by the wife of one of the professors of mathematics at Princeton, to be guest of honor at a tea. Reluctantly, Einstein consented. After the tea had progressed for a time, the excited hostess, thrilled to have such an eminent guest of honor, silenced the group. She turned to Einstein and said, “I wonder, Dr. Einstein, if you would be so kind as to explain to my guests in a few words, just what is relativity theory? "
Einstein rose to his feet and told a story. He said he was reminded of a walk he one day had with his blind friend. The day was hot and he turned to the blind friend and said,"I wish I had a glass of milk."
"Glass," replied the blind friend, "I know what that is. But what do you mean by milk?"
"Why, milk is a white fluid," explained Einstein.
"Now fluid, I know what that is," said the blind man, "but what is white?"
"Oh, white is the color of a swan's feathers."
"Feathers, now I know what they are, but what is a swan?"
"A swan is a bird with a crooked neck."
"Neck, I know what that is, but what do you mean by crooked?"
At this point Einstein said he lost his patience. He seized his blind friend's arm and pulled it straight. "There, now your arm is straight," he said. Then he bent the blind friend's arm at the elbow. "Now it is crooked."
"Ah," said the blind friend. "Now I know what milk is."