I made a Starbucks run with a couple of the girls from work one Monday morning after having spent the weekend doing a full blown machine polish and wax on my other car. When we got back to the car the girl holding the little tray of coffee for other co-workers at the office went to set her cup on the roof of the car. I told her to please not do this and she it was fine, don't worry, it won't scratch your paint. I told her I was less concerned about that and more that the cup wouldn't stay put and she'd end up wearing it. The slope was far less than 38 degrees, I can assure you (picture a spot about 3 inches toward the center of the roof above the door of a 4 door sedan - not much curvature). She looked at me funny so I said try it. She carefully let go of the cup and it started to slowly slide toward the edge of the roof. She picked it up and asked me how I managed to do that.

No flys were harmed in the making of that test. Not that I have a problem with that.