Basic Arithmetic

As a ten-year-old, I remember managing to sneak into a PG-13 movie (it was a Jackie Chan flick [probably one of the Super Cop movies], nothing too explicit). The person manning the theater relied on two things: honesty on my part and knowledge of mathematics. The first question they asked of course was how old I was. The second, which I found funny, was them asking me what year I was born. And perhaps every movie theater practiced this method for screening juvenilles that by the time they asked me the second question, I had already done the math in my head.