Got this from Car & Driver..
A not-too-bright but beautiful blonde was driving home one night when she was caught in a terrible storm. The hailstones were as big as golf balls, and her car was dented badly.
Next day at the auto shop, a repairman decided to have a little fun at her expense. "To fix the dents in the body," he said, "drive home, park the car, and when the tailpipe is cool, get down on your knees and blow really hard into the tailpipe, and the dents will pop out.
Later, a girlfriend of the blonde is driving by and sees her friend on her knees, blowing hard into the tailpipe. She asks what's going on and is told the story. The girlfriend laughs. "Well, duhhh! You need to roll up the windows first, silly!"
Grand Rapids, Michigan