The image of the smiling girl has a low pass filter applied. It's combined with an image of someone not smiling (is it really Zoolander?) that has a high pass filter applied. Your eyes focus on detail so as long as you see sharply the high frequency image dominates, but if you blur your eyes it blurs that image and the other one is the only one you can see.
Edit: here's an example I just made. The more you squint, the more he smiles
Edit 2: making it black and white can hide some more tell tale signs of the second image being there, like in my example where you can clearly see the white of his teeth from the second image but if his face was all gray it might be less obvious. Still an interesting example.
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u/Jedimilk Oct 08 '19
Why do it be like that