WHAT MAKES YOU LAUGH

My daughter sent me this joke and I fell in the floor laughing. Perhaps it was simply the timing. Anyway a few of my friends didn't get it or think it was funny. So here goes:

MENTAL HOSPITAL

I was walking past a mental hospital the other day, and all the patients were shouting ,'13....13....13'

The fence was too high to see over, but I saw a little gap in the planks and looked through to see what was going on. One of the patients poked me in the eye with a stick.

Then they all started shouting '14....14....14'.....