Optical Illusion
How many legs does the elephant have?
Optical illusion is an illusion caused by the visual system and
characterized by a visual percept that arguably appears to differ from
reality. Illusions come in a wide variety; their categorization is
difficult because the underlying cause is often not clear but a
classification proposed by Richard Gregory is useful as an orientation.
According to that, there are three main classes: physical,
physiological, and cognitive illusions, and in each
class there are four kinds: ambiguities,
distortions,
paradoxes, and
fictions.
Learn more on Wikipedia