I put up a video of this yesterday, Tuesday, but this video is shorter. In the first video I go into more detail and explain all the steps of the proof; here I give only the basics, only the bare bones.
Update (6-9-14, 4:15 PM): Takes a long time to get this done. Keynote/Powerpoint are hard to use: takes a long time to get the lines good. You make a diagram, copy onto the next few pages — then a line gets crooked: one side gets bumped up in the x by one, or down in the y by one; a line’s angle moves to 0.9º or 358.8º instead of being level; lines don’t match up as they did originally. Pain. Then you go to record audio and a timer you have set for dinner goes off, or a cat meows, or you misspeak half way through recording, or you, having retreated to a different room, find the recording is bad because, in that room, you sound like you are in a tunnel. Live and learn.