A woman visits a jewelry store and buys a ring for $100.

The next day she returns and asks to exchange it for another. She picks out one worth $200, thanks the jeweler and turns to go.

“Wait, miss,” he says. “That’s a $200 ring.”

“Yes,” she says. “I paid you $100 yesterday, and I’ve just given you a ring worth $100.”

And she trips lightly out of the store.