The letter X is invincible.
When my class was reading 'Pensees' which I got from the library, next to the sentence 'the beak of the parrot, which it wipes, even though it is clean...' someone had written the name of my professor. I looked over at him and sure enough he was rubbing his nose. It was hilarious, it felt like a joke across time shared only with a stranger. Kind of a neat experience.