Even chalk is inefficient though. One of the best math classes I had was taught on a projector with a wind-up transparent overlay. He'd use colored markers on t

Even chalk is inefficient though. One of the best math classes I had was taught on a projector with a wind-up transparent overlay. He'd use colored markers on the transparent sheet, and then wind up a little when he reached the bottom. If there was questions he could "rewind" to the point in question as nothing was ever erased. He then took the transparency roll to his office where it was available, in scroll-like form, during office hours.