[math-fun] decorated flexagon on the hong kong $10 note?

I just noticed that there seems to be a decorated flexagon on the hong kong $10 note. Here's a combined photo of the front and the back. They line up exactly when you hold the bill up to the light https://www.flickr.com/photos/thane/shares/zu9962 -- Thane Plambeck tplambeck@gmail.com http://counterwave.com/