Hands-on experiments with overturning some prisms (partially filled with water) lead students to a conjecture which can be confirmed by using a 3D geometry programme and reinterpreting the process of "overturning of a prism" in an appropriate way. But such confirmations are not a proof and particularly cannot answer the question "why" something is the case. Hence, a way is shown how the observed phenomena can be explained and proven in an elementary way, thereby also answering the question "why." This paper focuses on presenting ideas to deal with the topic in several ways: physical experiments using prisms partially filled with water, 3D geometry e.g. of GeoGebra, and establishing mathematical proofs.