The human brain works in mysterious ways. Optical illusions don't occur because of your eyes, they occur when your brain misinterprets information your eyes send it. An illusion is "a mismatch between the immediate visual impression and the actual properties of the object," according to vision scientist Michael Bach.
Everything that enters the senses needs to be interpreted through the brain, and sometimes your brain gets things wrong. So, what do you see when you look at the below image?