A friend of mine here at the station showed this to me today and it blew my mind!

"The Flashed Face Disortion Effect: Grotesque Faces from Relative Spaces" is a video created by Jason M. Tangen, Sean C. Murphy, and Matthew B. Thompson from the website PerceptionWeb.com and features dozens of perfectly normal human faces of various shapes and sizes delivered at a quick pace. I don't know how it works exactly, but after watching it a couple of times, my best guess is that various shapes and sizes of the face and the speed at which they change tricks your mind into thinking something's there that really isn't (hence the "optical illusion"). If you do it right, you'll swear after watching it that some of the faces had gigantic foreheads and bulging eyes or that a few of them were those sketches people draw after describing an alien abduction.

Here's what you do; simply expand the video to full screen and press play. Keep your focus on the cross in the middle of the screen. DO NOT focus on the faces on either side or the illusion won't work. If you think there's some kind of trickery afoot, watch it again and focus on the faces instead of the center cross and you'll see that other than the fact that there's a reason makeup was invented, none of the faces have been distorted in any way.

[Source: ebaumsworld.com]