An old man is selling watermelons by the side of the road.

His sign says:
1 for 3 dollars
3 for 10 dollars
A young man stops and buys a watermelon.
—That'll be 3 —says the old man.
The young man then buys a second watermelon. And then a third.
After paying another 3 dollars each time, the young man grabs his watermelons and starts to leave.
Then he turns around, smiling proudly.
—Hey, old man —he says—, do you realize I just bought three watermelons for 9 dollars instead of 10? Maybe business isn’t your thing.
The old man smiles and shakes his head.
—Funny... every time someone passes by here, they buy three watermelons instead of one... and then try to teach me about business.