Picasso was fond of drawing on napkins at some of the restaurants he visited. One day he was sitting at a table in a small Parisian cafe. A well-dressed woman approched and asked if he could draw a sketch of her on a napkin. 'You can charge whatever you want,' she said.
Picasso asked a waiter for two napkins. He pulled out his pen and drew a picture on the first napkin. On the second he wrote the astronomical price for the drawing. The woman gasped. 'Why is it so expensive? It only took you a few minutes to draw the sketch.'
Picasso thought for a moment, and replied, 'It took me a lifetime to learn how to draw that sketch.'
source: Book "The Lost Art"