The brain is often compared to a supercomputer. It can process an incredible amount of information in an unbelievably short amount of time. God specially designed our brains to instantly interpret pictures into a 3D world, even when they're 2D. This, along with other traits of the brain, has inspired many of the illusions that we love so much. Illusions that trick our eyesight, called optical illusions, take advantage of our brain's ability to fill in gaps about the visible world. We don't actually have to "see" everything to "get it." Our brain processes information quickly, even when the information is incomplete. Normally that is a huge advantage. When we're in familiar situations, our minds are especially good at making instant assumptions. Shapes and experiences from the past help form the way you think about the present. You are more prone to see what you expect.
Different types of illusions play off different patterns of thinking.
[Read the rest of the article at Answers in Genesis.]