I'd like to show you a very simple model that we built for explaining the kinematics of beams. That is to say how material deforms as a beam is loaded. This is just a simple piece of foam rubber on which we drew a line along the neutral axis. We drew transverse lines at equal spacing. And as we deform the beam, the center line shouldn't change, but it changes in length a little bit because of the nonlinear properties of the foam rubber. In fact, the top should shorten by the same amount that the bottom stretches, but because the material is a bit stiffer and tension than compression, those don't match exactly. You can even do more complicated geometries. And the students can see that as the beam deforms, the normals remain normal to the line of centroids, and those lines remain straight, showing that the planes remain straight.