That's the question I asked my mom when I was 6 years old.
Here's the full conversation:
Me: Mom? What's a number 6?
Mom: I don't know.
Me: Well ... you know, when you go to the bathroom number 1.
Mom: (Explained the finer points of the bathroom numbering system)
Me: Then, what is a number 6?
Mom: There isn't one.
Me: I think I just did one. I counted.
My mouth was like Russian Roulette. My mom never knew what would come out of it next.