As a wee tot, who had just discovered the power of words, particularly 'Why...?', I apparently learned that you could apply it to to everything, and expect an answer from your all-knowing parents. One day, my mum evidently got tired of explaining things to me, and said "'Cos" - i.e. "because..."
I must have been 'put out' by the fob off that next time she asked me why I had done something, I simply replied "Cos."
Well, after that, she didn't do that to me again.
The capacity young children have of absorbing all sensory input, particularly with regard to language, which they throw back again (often - amazingly - in the fitting context) was brought home recently when my sister visited my parents with her youngest son, who is two-and-a-half - and had recently been taught a naughty phrase or two by his impish older brothers (as they do), aged 6 and 8.
My dad was apparently cajoling him from outside the loo with playful regard to hurrying him up. "Hurry up, D," my dad called out. "Grandad wants to get in there." The next thing, my sister, who was in the small room with the little 'un, supervising his business, and my dad, were literally dumbstruck when a tiny voice called out in reply, "F*** you, Grandad."