Absolutely superb. And I'm doing an optical illusions special with my students this week too so this goldmine is perfectly timed. Thanks!

My favourite are the colour ones. I stare and stare at them trying to isolate the bit that - for example - is actually grey but looks red, to see its "true" colour. That level of concentration...not really ever seeing the grey, but perhaps objects behind me in the room flying around Matilda-style.