A colleague of mine told me today that that his girlfriend woudln't believe that there was an optical illusion in the animated dancer image that you can find at: http://dl.dropbox.com/u/2520452/ballerina/ballerina.gif
I was just about to play with html5 canvas drawing, and so I wanted to try out to pilot the illusion with a small red ball which can rotate in both directions, following an elliptical path (an illusion of perspective) and appearing to disappear when going "behind" the dancer.
I don't know if it works for all, but for me this simple (and unprecise) animation, combined with the psycologial effect introduced by clicking on a "change direction" button, swaps the dancer's direction immediately in my head, or at least within one rotation.
Click here to se a demo
Source code on github