Fred & Scott, Cambridge asked:
If you hold a sheet of paper at one edge so that it curls downwards, why does it then lift up if you blow across the top side?
If you blow air over a curved surface, it tends to stick to that surface; it’s called the Coanda Effect. If the surface, in this case the paper, is bending downwards, it will pull that air downwards with it.
If you push something, it pushes you so if the paper is pushing the air down, the air will push the paper up. This is exactly the same principle as is used in aeroplane wings; they push air downwards, so the air pushes them up.