As therapists every day we help our clients to challenge what they think they see – helping them to see that the reality they think they see is often created by them and is actually contrary to the actual reality. There is no better way to illustrate this than with optical illusions. Here is a pretty good one.
There is a YouTube video of it: http://www.youtube.com/user/brusspup
And also a scientific paper discussing it: http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html