John had a new car, but it was in the garage for repairs, so he borrowed his wife's car one evening. He found that it didn't have much gas in the tank, so he drove to a service station and filled it up. But then the car wouldn't start.
He thought there was probably something loose in the battery, so he took a small wrench out of his pocket and it the battery with it. The car started at once.
Then John saw that one of the lights was not working either. "There's something loose there too," he thought. He hit it on the side with his wrench, and it lit up right away.
The garage man was watching, and then he ran over to John and said, "If you ever want to sell your car, I don't want it, but I'd be very happy to make you and offer for that wrench."