QI is a really rather wonderful, charming, comedic, and quite interesting show. The clip above is taken from the most recent series (series 9, the I-series), and is a fantastic example of what goes on in QI.
The actual optical illusion is amazing as well, especially considering it works every single time! Even when you know what to look out for, your brain still fools you. There are many examples of this effect, and one which you can make yourself is seen in this clip: Amazing Dragon Illusion. It’s so simple, and yet so effective.
You’re twisting my melon, man! Onwards!