An all-day pass for a mini-locomotive at a leisure park costs 1 Euro, and a single-ride ticket costs 50 cents. A woman unknown to the driver enters the train ride, gives him 1 Euro without saying a word, and receives an all-day pass.
How did the locomotive driver know that the woman wanted an all-day pass and not a single-ride ticket?
The woman paid for her train ticket in small coins. She would have been able to pay exactly 50 cents, too, if she had wanted a single-ride ticket.