Someone asked Albert Einstein at a party:  "Oh, you are Albert Einstein, could you please explain me your relativity theory in three or four sentences?" 

Einstein thought for a while, then he said, "I am very sorry, it is not really possible to explain relativity theory so quickly, but I will at least try to explain you why this is so. 

Once I went for a walk with a blind man on a warm summer afternoon.  After a while we came to a restaurant and sat down at a table.  I asked him, 'Would you like a glass of milk?'  'I know what a glass is,' the blind man said, 'but what is milk?' 

'Milk is a white fluid,' I replied. 

'Aha, I know what a fluid is,' he said, 'but what is white?' 

'White is the color of swans,' I explained to him. 

'I know what a color is, but what are swans?' 

'Swans are big birds with a crooked neck.' 

'Oh, birds,' the man exclaimed, 'I have heard of birds.  And what is crooked?' 

I took his arm, stretched it and said, 'This is straight.' Then I bent it and said, 'and this is crooked.' 

The man's face turned into a happy smile and he said, 'Now I finally know what milk is.'"

                                             End!