See the two faces? Good.
Now step back from your computer and move away while looking at them. See their expressions change? Do you now doubt your senses? Good.
Aude Oliva and Philippe G. Schyns created this illusion to mess with your head. It takes advantage of the fact that fine detail becomes indistinct at distance so we’re forced to use more general cues to determine expression and that can mess with our interpretation of what we see.