Hollow Face Illusion
This is quite a well known illusion. Basically, a hollow mould taken from a face actually appears to be a three dimensional convex face, even though it is actually concave.
If you search on the web, you can find quite a lot of information about this particular illusion. A good article comes from Richard Gregory - http://www.richardgregory.org/papers/knowl_illusion/knowledge-in-perception.pdf - very appropriate really, since it is his face that we are seeing in this picture.
Tim has got hold of this amazing hollow face of Richard Gregory, Professor of Illusions at Bristol University, which he demonstrates in the video. It is twice life size, and has been painted as well.