An optical illusion is when visual perception is different from what the actual reality is. The interpretation of an object or situation is processed in the brain in a different way than the physical stimulus should appear. In Gestalt theory, it is thought that the brain will take individual stimuli and try to construct a meaningful whole picture of the situation. A famous example is the reversible figures and vase image or duck rabbit illusion. Another point of view from evolutionary psychology is that in order to gain a survival advantage, its important to see the whole scene in terms of the edges and that the brain will try to make sense of symbols and shapes to formulate an entire picture, which may not be what the reality actually is. Sometimes, parallel lines that converge can fool the brain that the image higher in the field is farther away and the brain perceives it to be larger. This can be seen in a Ponzo illusion. Sometimes, in a moving car, stable surrounding objects seem to move or even move backwards. If you have successive pictures that are slightly varied, as in a children's flip book and and you flip it, the brain sees it as a moving picture. These are many examples of why visual perception is not 100% reliable. That is why in science, we use tools to measure and collect data to gain an accurate insight into the reality of a situation.