When the mind receives visual information, it will fill in details or gaps that do not actually exist. The eye and brain will choose and focus on specific objects which causes on part of the image to appear one way or the other. Depending on what the brain chooses to focus on, it can perceive two different images in one.
The picture of the elephant is a standard example of a literal optical illusion. The artist only drew the elephant's legs without feet and then feet without the elephants legs. However, since our mind uses edges to identify objects, it sees more than four legs. This creates a confusion with our understanding that elephants only have four legs.
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