A famous art collector is walking through the city when he notices a mangy cat lapping milk from a saucer in the doorway of a store and he does a double take. He recognizes that the saucer is extremely old and very valuable, so he walks casually into the store and offers to buy the cat for two dollars. The store owner replies “I'm sorry, but the cat isn't for sale.”
The collector says, “Please, I need a hungry cat around the house to catch mice. I'll pay you twenty dollars for that cat.”
And the owner says “Sold,” and hands over the cat.
The collector continues, “Hey, for the twenty bucks I wonder if you could throw in that old saucer. The cat's used to it and it'll save me from having to get a dish.”
And the owner says, “Sorry buddy, but that's my lucky saucer. So far this week I've sold sixty-eight cats.”