<a href="http://www.youtube.com/watch?v=fQEHtvNsfKE" rel="nofollow">http://www.youtube.com/watch?v=fQEHtvNsfKE</a><p>I think projection, with a contact lens with some sort of polarizing effect so that you have to be looking right at it to see what is being projected is the ticket. However, who knows if that is even possible.