One of my pet peaves is how many web sites and books use "hump" theory to explain lift: "the hump on top of a wing makes the air travel further to catch up to the air below and faster moving air has less pressure". For example, older versions of Microsoft Encarta used "hump" theory, until the 2002 and later versions. You can still find a lot of web sites that still refer to "hump" theory. I work with a group of programmers / engineers and I remember a co-worker with an instrument rating that believed in hump theory, as well as other co-workers, and it took a bit of convincing to get them to change their beliefs. It's a two step process. Step 1, explain how pressure differential is how air exerts it's weight inside a container. For example, putting 80 cubic feet of air into a scuba tank increases weight by 6 pounds, and it's the pressure difference versus alititude within the container that causes the net downwards force exactly equal to the weight of the air inside the container. Step 2. Replace 1 pound of air with 1 pound model aircraft. It's a closed system, so as long as the center of mass isn't accelerating vertically, the total weight of the system never changes, regardless of what the model is doing, as long as the center of mass of the model has no vertical component of acceleration. If the model is flying within the container, then it has to increase the pressure differential by just enough to create a net downwards force within the container equal to the models weight. Therefore the model lifting surfaces are air pumps creating a downwards flow of air that results in the net increase in pressure differential within the container. There's a third step to explain how a lighter than air balloon works inside a container: Step 3 - If the model is a helium baloon, then as the balloon is inflated within the container, it increases the pressure within the container, which increases the pressure differential, just like adding more air which would add more weight and increase the pressure within a container. If the balloon is hovering,then the net increase exactly compensates for the weight of the balloon. If the balloon is pushing against the top, the net increase in downforce due to pressure equals the weight of the model balloon plus the upwards force the balloon exerts on the top of the container. If the balloon is pushing against the bottom, net increase in downforce due to pressure equals the weight of the model balloon minus the downwards force the balloon exerts on the bottom of the container.