Do you remember the story of Dr. Jekyll and Mr. Hyde? Dr Jekyll, the famous scientist, is not all he seems, and the other side of his character - Mr Hyde - is, you will recall, a distinctly nasty piece of work.
Continuing the idea of the two sides of the human character, have a look at the picture above. On the left is Dr Angry, and on the right is Mr Calm.
Now, back away from your computer screen. This effect will work at different distances for different people, but you should see the two characters swap over...! As you come closer to the screen again, they revert to their original characters. Spooky or what!
As optical illusions go, we think this is pretty impressive. A short explanation goes something like this. When we look at an object, we can normally see both fine detail and coarse detail. However when we are close, the fine detail will dominate, and when we are further away, we lose the fine detail, and see more of the coarse detail.
Both of the faces you see above are hybrids - each face is actually a combination of two faces. The left hand face shows an angry man in fine detail, but within the picture there is also coarse detail of the calm face. Move away, and you lose the fine (angry) detail, and just see the coarse (calm) detail.
The right hand face shows the calm face in fine detail, and the angry face in coarse detail.
This is based on work by Dr Aude Oliva (MIT) and Dr Philippe Schyns (University of Glasgow). You can read the original paper here