are cool and all that, but typically, what happens is you are shown a nondescript diagram made up of simple lines or shades of gray, and asked to determine which line is longer or whether it’s straight or which shade of gray is darker or lighter, and then told that, ha, gotcha! It turns out they’re the same length, both of them are straight and yes, they are the same shade of gray.
But optical illusions sprung to life for me when I came across the Predictably/Irrational web site. Check out the portion of this page (scroll down a bit) that demonstrates optical illusions. After showing you those nondescript diagrams and asking you those questions about the lines and the shades, the animations go on to show you, in slow motion, how your eyes and your mind are the unreliable measuring tools that they are. Check out this I-can’t-believe-my-eyes Koffka Ring demonstration, for example. Or this Jastrow illusion.