They took a limo to JFK airport. When they arrived, she got out of the limo. She said, "I already paid the driver to take you back home. I've paid for everything." An hour later, Melvin gave the driver $20, saying, "Thanks. Here's your tip." The driver thanked him. Melvin entered his apartment and called his wife. She was still waiting to board her plane. He said he had tipped the driver $20. She said, "Why? I had already tipped him! I told you I had paid for everything." What a greedy driver, Melvin thought. He had accepted two tips for one ride. A month later, Melvin called the driver to get another ride to JFK. The fare alone was $50. When Melvin and his wife arrived at JFK, he gave the driver $50 exactly. Melvin said, "Remember last time? You accepted two tips for one ride. Now we're even."