Revolution Volumes
Solids obtained by rotatiing a region around an axis can be computed by two different slicing methods: one is the washer method, the other is the shell method. This depends on how the region under consideration is sliced up. We teach this in single variable calculus courses. It is however a multi-variable calculus topic and actually hits there also one of the most difficult concepts to grasp: the change of the order of integration. Additionally, we have then to grapple with the fact that we rotate stuff around an axis which is hard to wrap your head around. Your head starts spinning. Words like "washer" and "shell" help to get grounded here.