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A man lives on the 21st floor in a high-rise apartment complex. Each work day, he takes the elevator down to the ground level, catches a taxi, and goes to work. At the end of the day, a taxi brings him home. He takes the elevator up to the 18th floor, and uses the stairs the rest of the way up to the 21st floor. If it is raining, he comes back from work and takes the elevator all the way up to the 21st floor. Explain this odd behavior.