I’m sure you recognize the subject of the photos, but the point of this has nothing to do with his identity. The question is, without turning your head around to see these right-side-up, what differences can you see between the two photos? Are there indeed any differences, or are they identical? Take a moment to ponder all of that, then…
There was an explanatory bit of text accompanying this that I won’t include, since it was in a magazine and not online where I could simply link to it, but if you’re really curious, it was in a special edition magazine titled Scientific American Reports: Special Edition on Perception, which came out earlier this year and is chock-full of such images, along with some fascinating explanations.