There are optical illusions all around us. They happen naturally. One example is a mirage. On a hot day, it looks like there is water on the road when there really isnít. This is because the light is being refracted by the hot rising air. But our eyes are fooled into seeing a puddle.
Many animals depend on illusions to survive. Their bodies camouflage into their surroundings and they are hard to see to their predators. An example of this is the lion. He blends in with the grassy plain that he hunts on in order to hide, while the zebra has stripes to fool the lion into not knowing one zebra from an entire herd.
Insects also use camouflage to protect themselves. The walking stick looks like a twig to avoid being attacked and there are many butterflies that appear to have "eyes" on their wings to scare off a predator.