Charlie Chaplin rotating mask
This astounding illusion was first described by Richard Gregory. It’s a form of depth inversion and it involves a hollow (concave) object which appears to be non-hollow (convex).
Initially you see Charlie Chaplin’s face on the outside of the mask. However, as the mask rotates, your visual system refuses to see the inside of the mask as a ‘hollow’ face. A lifetime of experience has taught you that faces always stick out. So despite the fact you know the mask is hollow, your visual system dismisses this hypothesis as too improbable, and favours the hypothesis that the hollow mask sticks out.
As the mask rotates, and the Charlie Chaplin face disappears a particularly interesting effect is seen and felt. There is a moment of slight confusion, before the inside of the mask is perceived as sticking out; at this point the mask appears to rotate in the opposite direction.
In a paper called The hollow-face illusion Hill and Johnston discussed the intricacies of the mask illusion, concluding that probably it’s driven by expectation, but it also relies on their being some ambiguous information in the first place. And while the expectations in question don’t need to be about specific objects, like faces, it helps if they are.
Oh, one last thing. If you can’t see this illusion, maybe, you are in trouble.