A Mobius loop (double perf 16mm loop with a twist so it switches left and right with each revolution) in which you can easily see the flip. One of the ongoing "Blueballs Suite" series, starts with peeling the color varnish off Xmas ornaments, then adhesive taping this onto black leader in this case, which had been hole-punched (and each hole is punched through two layers of film to further maximize the cyclical loopiness of it all).