Distance-based face-perception illusion (pic)

[Update: a copy of the illusion can be found here, from this page containing many examples of optical illusions.  Some details of how the illusion works can be found here]

Have a look at this very cool visual illusion. On the left is the face of an angry man, on the right is the face of a calm woman. But get up, and walk three or four meters from your screen and look at it again -- the image is exactly the same but the faces have swapped positions. Walk back towards the screen and see them magically switch again.