Hexaflexagons [video] (2012)
I saw a reference to Martin Gardner's article when I was in, maybe, 9th grade.<p>So I went to the main branch of the NY public library ("reading between the lions") and looked up the article on microfiche. It was fascinating and fun - from how to make a row of equilateral triangles with nothing more than a ruler, pen and paper. Many, many hours were spent on the various flexagon derivatives.<p>Years later, I made a JS page to combine images and print hexaflexagons for my son's classmates. Had some fun with that as well; it was much more confounding than I expected.<p>Nowadays, microfiche is gone, the public library is unfortunately not in most kids vernacular, and the attention span that would be required to enjoy doing the math I did has been hijacked. (TikTok !== figure-out-flexagon)