At a social gathering, the hostess asked Einstein to explain his theory of relativity....
Einstein: Madam, I was walking in the country on a hot day with a blind friend, and said that I would like a drink of milk.
Blind friend: Milk? Drink I know; but what is milk?
Einstein: A white liquid.
Blind Friend: Liquid I know; but what is white?
Einstein: The color of a swan's feathers.
Blind Friend: Feathers I know; what is a swan?
Einstein: A bird with a crooked neck.
Blind Friend: Neck I know; but what is crooked?
Einstein: Thereupon I lost patience. I seized his arm and straightened it. "That's straight," I said; and then I bent it at the elbow. "That's crooked."
Blind Friend: Ah! Now I know what you mean by milk!