Why are they chanting???

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A guy is walking past a high, solid wooden fence at the insane asylum and he hears all the residents inside chanting, "Thirteen! Thirteen! Thirteen!" He continues walking along the long fence, but, being a curious person, he can't help but wonder why they are chanting "Thirteen!" over and over. Could it be that they are chugging beer? Are they perhaps taking turns beating one of the inmates? Maybe they are counting the number of patients that have leapt off of the roof thus far. His curiosity peaks and he frantically searches for a hole in the fence so that he may see what is going on. Finally, he spots one a few feet ahead. The hole is low in the fence and he has to kneel down to peer inside. He moves into position and peeks into the hole. As he looks in, someone inside pokes him in the eye! Then everyone inside the asylum starts chanting, "Fourteen! Fourteen! Fourteen!"