Don't know if you can pull this off, but it was of the funnier prank stories I heard on Klick and Klack/Car Talk. Some guys worked with another guy who was obsessed with his car's gas mileage, so the coworkers slowly started adding gas to his tank without the owner knowing it, like they'd add two cups a day for a week, then the next week, they'd add three cups a day, etc. Since the guy was so obsessed, he thought his car was getting better and better gas mileage and was bragging to everyone in the office. Then the guys slowly started to reverse the process, the guy freaked out, everyone laughed.
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