Thinking out of the box

An old man who sells watermelons had his pricelist read: 1 for $3, and 3 for $10

A young man stopped by and bought 3 watermelons one by one paying $3 for each

As the young man was walking away, he turned around and said: “Hey Oldman, do you realize that I just bought 3 watermelons for $9 instead of $10? Maybe business is not your thing!”

The old man smiled and mumbled to himself, “People are funny. Every time they buy three watermelons instead of one, yet they keep trying to teach me how to do business!”