How do optical illusions work?

(Inside Science TV) – These are two identical, straight lines. Kind of boring, right? But what happens when you change what's going on around them? All of a sudden, the lines look like they've changed size in relation to each other; they could shrink and grow; they might change to different shades; they may stretch and compress; and they could bend, buckle, or even change directions.  But even though it would seem like the lines themselves are changing, the only thing that's actually bending or buckling is your mind – the truly mesmerizing part of optical illusions.