Optical illusions work because humans have binocular vision, which allows people to see different objects with each eye and sometimes the images do not merge properly. The brain must also interpret the signals correctly. Persistence of vision can also make optical illusions work.
Optical illusions can be fun to play with. When people design optical illusions, they count on the way the eyes work to make the illusion work.
Most humans have two eyes that are located about two inches apart. Most times, the brain combines the two images to create one, but sometimes in optical illusions this does not happen.
When the brain receives an image from the eyes, it wants to make it a familiar object. Sometimes, optical illusion designers rely on this fact to make their illusions work.
After the brain has received optical stimulus, it still thinks the person is looking at that same thing for a few milliseconds even when the eyes are taking in additional messages. Optical illusionists sometimes use this fact to create their designs. It is the reason that flip books work.