I have heard so many different explanations of why planes fly that I doubt that anyone really understands it. There was a memorable moment towards the end of my engineering degree, when after being given yet another explanation of why planes fly a classmate shouted out, "I think we've heard enough theories about whether planes can fly".

Apparently chemists encounter a similar thing when studying chemical bonds - every new academic year you are told that what you learned the previous year was such a gross simplification that it's basically wrong.