I can see how it's impossible. If the top and right faces are planar, the front face can't be. It becomes possible again if you cut the front face into two tri

I can see how it's impossible. If the top and right faces are planar, the front face can't be. It becomes possible again if you cut the front face into two triangles, that being the "hidden edge" the author mentions. The pyramidal explanation sounds like it's using a theorem that I didn't know before, but without proving it. You could say the explanation deserves its own explanation. It's an interesting fact, that the lines have to intersect, nonetheless.