Theorem and proof
As a teacher, the best way to prove an idea by showing your students actual proof. You can conduct all sorts of science experiments and whatnot, but sometimes, all you need to do is roll up your sleeves and put your money where your mouth is.
Or, in this teacher’s case, put your fist where your mouth is. He told his students that he had a massive mouth, to which the students were confused as what they could do with this information. To make a point that nobody asked for, he stuck his fist into his mouth, thereby proving his theorem to his audience of laughing students. We need more teachers like this man.