An old man is selling watermelons. His pricelist reads: 1 for $3, 3 for $10

A young man stops by and asks to buy one watermelon. “That’d be 3 dollars”, says the old man.

The young man then buys another one, and another one, paying $3 for each.

As the young man is walking away, he turns around, grins, and says, “Hey old man, do you realize I just bought three watermelons for only $9? Maybe business is not your thing.”

The old man smiles and mumbles 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…”