I added an old hat to my collection of  #opticalillusion   http://michaelbach.de/ot/cog_blindSpot/

When you get people to perceive their blind spot (or lack of perception there) for the first time, it amazes everyone.

A tip for boring presentations: close one eye, and try to loose the head of the presenter in your blind spot – can lighten up the most wearisome sessions :).