Why does your brain get fooled by optical illusions?

Our Brains are both "Wired" and "Trained" to respond in specific ways to Optical stimuli. For instance, our Brains use "Perspective" (size in particular) to determine when an Object is near, or far away.

So we are easily fooled (initially) by a picture in which two similar Objects appear to be different sizes. We assume that the smaller Object is farther away than the Larger one!

Other factors that tend to fool the eye are Angled Lines, Arcs, and Tapers. Our Brains make assumptions and take shortcuts to process the huge amount of information that comes into it through our senses. Sometimes this causes us to make mistakes in perception...fall for an Optical Illusion.