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](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](http://dl.dropbox.com/u/2520452/ballerina/ballerina.html)
