Optical illusions have a way of driving people crazy. No matter how much you try to avoid being “tricked,” your brain often interprets things in unexpected ways. One illusion now confusing the entire internet shows six girls sitting on a couch—yet only five pairs of legs are visible. How is that possible? According to the Queensland Brain Institute, an optical illusion happens “when our brain and eyes try to speak to each other in simple language but the interpretation gets mixed up.”

Our brains evolved to react quickly to movement for survival, so sometimes they misread what our eyes see. A viral Reddit post features the now-famous photo: six girls lined up on a couch, but only five sets of legs in view. At first glance, it looks as if one girl’s legs have completely disappeared, sparking endless debate online.