Suppose that a friend asks you to drive him to the airport this weekend so that he can catch a flight. He pays you for the gas used driving to the airport and for the cost of parking the car at the airport while you help him in with his bags. He then declares that you are now even, since he has fully compensated you for any costs you incurred in helping him get to his flight. From your perspective, what aspects of the price of taking your friend to the airport has he omitted?