Your brain's construction project
Stare at a simple wire-frame drawing of a cube, and something peculiar happens. Without any change in the image itself, your perception of its orientation will spontaneously flip. The face that appeared to be at the front suddenly becomes the back, and vice-versa. This is the Necker Cube, an ambiguous figure that shows a fundamental principle of vision: we actively construct reality, we don't passively receive it. The drawing on the page is a flat, two-dimensional object, yet the brain insists on interpreting it as a three-dimensional cube. Since the line drawing offers no cues for depth, two equally valid interpretations are possible, and the brain alternates between them.
The phenomenon was first documented in 1832 by Swiss crystallographer Louis Albert Necker. He observed the effect not on a purpose-drawn cube, but while examining drawings of crystalline forms, noticing a "sudden and involuntary change in the apparent position of a crystal or solid represented in an engraved figure." This bistable perception, where the brain switches between two stable interpretations of a single sensory input, shows our visual system is a dynamic process of hypothesis testing. It makes a "best guess" about the structure of the world and, when faced with ambiguity, it will test out the alternative.