This illusion is great.  Just follow instructions and be amazed. I love this sort of thing.  It's a demonstration of the brain mis-firing, and it mis-fires for us all.  There's a reason why eye witnesses disagree on almost everything they see with their own eyes.  The eyes see what they see, and the rest is done by the brain.  And the brain is not a perfect instrument.  It has to interpret everything based on many, many factors.  I don't know...