Nicely done. It doesn't look like a surprising theorem at first sight, though. The wikipedia section on Generalizations does make it a bit more surprising.
Nice visual demonstration, I had never heard of this theorem before, but it seems to visually show that any polygon is made out of triangles, any triangle can be turned into a rectangle with a chosen width, those rectangles can be combined into a larger rectangle, from the larger rectangle you can extract other triangles, from which you can make the other polygon...