Illusions are fascinating. Like brain injuries allow us to understand brains, illusions - cognitive or optical - help us understand the nature of our connection with reality. One fact is outstanding: once you are aware of the trick presented to you, you can switch the illusion on or off at will.
Here's a very cool example where they slowly build up to the illusion: as soon as you get it you can switch it on or off.
Neat. I'd like to see it with people walking what looks like down the side.