the amazing adaptability of the human body. a strength as much as it is a weakness. theres another really cool one where there are two identical pictures next to each other, but one is purple and the other looks normal. if you stare at the purple one, then quickly look over to the normal one, the normal one will appear bright yellow (or green) for several seconds, because in that short amount of time, your brain actually adapts to that intake of purple light being "normal", and when you look at the different scene it takes a moment for your brain to switch back to white light being "normal", and you see the scene in a different, "compared to purple being normal", way.