Flip a distorted face upside down and the distortion disappears

Psychologist Peter Thompson rotated Margaret Thatcher's eyes and mouth 180 degrees within an otherwise normal photo of her face, and the result looked monstrous — but only right-side up. Turn the whole photo upside down and the grotesque swap becomes almost invisible, because your brain stops reading faces feature-by-feature once they're inverted. It works on nearly any face and is now called the Thatcher effect.

— Peter Thompson, Margaret Thatcher: A New Illusion — Perception, 1980