Optical illusion
The general explanation for most illusions is the way the brain works on sense data to produce a meaningful perception. A great 19th century psychologist Hermann von Helmholtz described perception as "unconscious inferences from sensory data and past experience". Richard Gregory discussed how the brain makes an hypothesis about what's out there, and sometimes the hypothesis does not quite work out. His model of perception is an interaction between data from sense organs and previous knowledge and experience.