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Now look at it again, tilting your head upside down so you can see the lady upright.
It becomes immediately apparent what's different between them. Even after seeing that, when you look at it normally again the images still look very similar. Now you might notice a little difference in the position of the pupils in the eyes. Optical illusions are like that. Even after you've been shown the illusion, it's like you haven't learned anything.
The image is a self-portrait of Taryn Simon. She used it as an example in her TED conference speech.