Kanizsa Triangle

The Kanizsa triangle is an example of illusory contours: when a drawing purposefully leaves certain areas blank, our brain tries to perceive a geometrical figure in them (in this case, a triangle), making up the missing contours.

While being the cause of the deceiving illusion, the ability of seeing figures even when they are not explicitly drawn is a basic capacity for geometrical thinking: a person lacking this ability would probably not be able to solve even the most basic geometric problems.

The Kanizsa triangle: our brain perceives a white, bright triangle in the foreground, even though the contours are imaginary