I was wondering about the origin of using the terms "number one" and "number two" for going to the bathroom (for those unaware, number one is urinating, number two is defecating, at least in the US). I have used it several times myself without actually thinking about it and heard plenty of others use it as well.
A quick Google search showed some interesting yet conflicting results. This site mentions an old school room signal that children would use to ask to go to the restroom (holding up 1 or 2 fingers). But then a post further down claims that is just an urban legend and explains why it is unlikely to be the origin.
I used Google Ngram for "to go number one" and "to go number 2" and it looks like a huge spike started in the mid-to-late 70s. (For jokes I checked "to go number three" and " to go number four" and got no results, I know I just saved a few of you some time!!!).
So, where did the expression originate?