You probably think that a wing lifts an aeroplane because the airflow moving over the top has a longer distance to travel and "needs to go faster to have the same transit time as the air travelling along the lower, flat surface". Well, you are wrong.
University of Cambridge's Professor Holger Babinsky was so frustrated that everyone seems to believe this that he thought he should demonstrate that it is absolutely wrong:
I don't know when the explanation first surfaced but it's been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics.
So he created this one-minute video that actually shows what happens to the air as it travels the wing. It's a quite simple and clever demonstration of how air actually goes around the wing.
What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes -- lower pressure on the upper surface and higher pressure on the lower surface. This is why a flat surface like a sail is able to cause lift -- here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it's the curvature that creates lift, not the distance.
There you have it. It's the curvature, not the distance. He hopes the video will debunk that stupid myth once and for all.