duty

"If someone says the word "duty", I can't help but laugh."

What is it about the potty humor that holds us throughout our lives... it is really the first humor kids seem to get (that and slapstick)... yet we never get over it... I got a bulletin from a friend with a list of things... above being #12 and when I read it i laughed so hard it hurt. every time i see it i giggle...
When i watch the news and some one mentions Putin i laugh without fail which causes me to lose track of whatever news story I am trying to follow.
The other day my daughter and i were watching something with a butler... she realized that butler had the word but in it and it was all down hill from there. it gave way to endless butler jokes...yesterday all she could talk about was rodney copperbottom from the movie robots and she laughed and laughed... i can't recall the last time she saw it but now it's "copperbutt" or "copperbootie" once even "copperass" though I put a stop to that one. Today she begged to watch Madagascar so she could hear the monkey talk about flinging his poo... and it isn't only her... or me.... or even mike for that matter. I started to question if maybe we just live in a potty-centric house... but no, we're living in a potty-centric world...
In truth I find nothing funny about bodily functions. I don't want to see anyone shit piss or vomit... but make a joke about it and i'm gone...
where did we get our love of the duty joke?