So I've been trying to prepare a long-planned end-of-term lecture on optical illusions with lots of fun things.
Only trouble is, every time I look at the examples I've got, I get a blinding headache. So now I'm going to have to improvise tomorrow because I don't want to risk what might happen when these things are projected 20ft across in a darkened room.
If you want to risk your own health, there's a great collection of illusions like the one above (along with explanations of how they work) at Micahel Bach's site. But be warned - I really am suffering ;-)