Milestones not found in the baby book

Yesterday as I was doing some catching up online my seemingly innocent little three year old girl was in the kitchen. I then hear her curse for the first time, quite curtly. It was the s-word that basically means crap. And I don't mean Spam. After a second of slight shock, I reprimanded her then asked her what it was about. She informed me that the baby had spilled Cheerios under the kitchen table. I said "oh, ok, but you still shouldn't say that" then checked to see if she was looking and broke out into silent laughter.

She not only cursed, but used it in proper context and everything. I don't know whether to be mortified or proud.