I think most people like optical illusions. We take what we see so much for granted most of the time that to have a specific demonstration of how easily we are fooled is rather striking.
The best I have seen for a long time is provided on the Bob Atkins Photography site where he shows the image below and then tells you that the squares marked A and B are EXACTLY the same colour/shade. No matter how much you study this photo, you cannot convince yourself of this.
You need to visit the site of Dr Adelson, who created this image, to get an explanation of what is going on.
The image below with a grey line through it does demonstrate that the two squares are the same.
Other than the addition of the line, the image has not been changed. It does help the eye/mind see things a little differently.