Psychology
One drawing is either a vase or two faces, and your brain won't hold both at once
Danish psychologist Edgar Rubin's 1915 vase illusion showed that perception constantly assigns regions of a scene to 'figure' (the object you're looking at) or 'ground' (the background), and an ambiguous image can flip between two readings but never be seen as both simultaneously. It became a founding demonstration of Gestalt psychology's claim that perception organizes raw sensation, rather than simply recording it.