Hey Mike,
Do you happen to teach Geometry in the real worl? Just because you and I understand what you just wrote, doesn't mean very many newbies will. []

rider,
Minnflyer's is a very nice, detailed answer to the question. I hope you will take the time to read it, and try to really understand it. Use your hand to act out the explanation, and try to visualize the air moving under the surfaces.
Dennis-