英语笑话:Thirteen!

Thirteen!
　　A guy is walking past a big wooden fence at the insane asylum and he hears all the residents inside chanting, Thirteen! Thirteen! Thirteen!
　　Quite curious about this, he finds a hole in the fence, and looks in. Someone inside pokes him in the eye.
　　Then everyone inside the asylum starts chanting, Fourteen! Fourteen! Fourteen!