Two pictures with the exact same face. But, in one photo you see a girl and in the other one - a guy.
The fact is that this face was created on computer by mixing male and female face features, which gave this androgynous face. With a change of contrast, our brain recognizes the face on the left as being a female one, while on the second photo, where contrast is higher, our brain recognizes it to be a male’s face. For me, this is the best optical illusion of the century.