The "impossible" part is trying to imagine a "flat" ADFC face. There is no such "ADFC" face that can result in a triangle in the shape of DEF. ------- To ima

The "impossible" part is trying to imagine a "flat" ADFC face. There is no such "ADFC" face that can result in a triangle in the shape of DEF. ------- To imagine what "ADFC" could possibly look like, I suggest trying to visualize the ABC-base at first, and then trying to imagine different "ADFC" faces to match the ABC-triangle with the DEF-triangles. The only way you can get this shape to look like this, is if ADFC is bent or twisted somehow. No "straight/flat" ADFC could possibly connect the bottom with the top.