The Muller-Lyer illusion;
Lines A and B are the same length, yet look different – why? The answer is that you have learned to process this kind of stimuli in a certain way. We live in a “carpentered world” of straight lines and we interpret line B as a corner (therefore larger than it appears, because it must be far away) and line A as a corner (therefore, smaller than it appears, because it must be close).
These optical illusions do not work on Zulus, which proves the illusion is caused by learned visual processing, rather than an innate function of the eye / brain.