There appears to be no universally accepted rigorous definition of a "flexagon" (although I will try to give a reasonable one in the talk). Examples of flexagons were most likely discovered and rediscovered many times in the past - but they were "officially" discovered in 1939, a serendipitous consequence of the discrepancy between US paper sizes and sensible paper sizes.* I'll describe a couple of the most famous examples of flexagons (with actual models to play with of course), and also introduce some of the more abstract theory of flexagons which has been developed. Feel free to bring your own models of flexagons!
* The views expressed herein are solely those of the speaker, and do not reflect the official position of the Kinderseminar w.r.t. international paper standards.