A very effective didactical way to stimulate students to seek the correct interpretation of physical phenomena beyond the appearances is to realize certain devices whose way of working manifests behaviors different than those that anyone would expect and that's the reason why they are callen paradoxes. But any law of physics is violated: it is simply used in a bizarre way, deliberately amazing. So this particular intruments are able to capture the viewer's attention, to whom a little effort it's required to resolve the paradox knowing the laws of physics.