Reasoning

A guy goes into a café, orders a pastry, and sits down. A few seconds later he brings the pastry back to the counter and asks to exchange it for a glass of liqueur. The proprietor agrees, the man goes back to his seat, slowly finishes his drink, and gets up to leave. “Hey,” says the proprietor, “you can’t leave until you pay for your drink.” “But I exchanged my pastry for it.”
“You didn’t pay for the pastry, either.”
 “But I hadn’t eaten it.”