The wisdom of a five year old

Our second daughter asked me today why we have swear words if we can't use them...

It does recall a certain incident a year or so ago, where said daughter knocked over her glass of water, turned round, and said through clenched teeth f**k, f**k, f**k, f**k, F**K! This was observed by my dear wife, who pursued me as to the origin of this quaint English phrase. I denied all responsibility, at least until the next time I did something similar, and, unbidden and completely to my surprise, guess what came out of my mouth...