For as long as I can remember the anwer I've heard to this question was the "Equal Transit" theory - that since the top of the wing is longer that the bottom the air has to go more quickly over the top inorder to "keep up" with the bottom causing a lower pressure on the top than the bottom. But this doesn't really make sense - why should the air on top "care" about the air on the bottom? When I looked on Nasa's website I found a different explanation: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html and a debunking of the "Equal Transit" theory: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html So why have I never come accross the explaination Nasa gives and everyone gives the "Equal Transit" theory? Which explanation is true? Or is it both? Thanks.