I was recently driving around in my gas guzzler running errands, and I was listening, as I always do from 9 to noon on weekdays, to Glenn Beck. I don't recall what the topic was, but at one point in the program he used the word 'ass.' I thought, 'ooooh crumb, hope the Boy didn't hear that...' This hope was dashed however, as I heard my son very matter-of-factly say from the back seat, "Ass means butt."
At this point I had to do the mom-smother, which is where we cover our mouths to keep from busting out laughing when our kids say something really funny but totally inappropriate. I then gave my best mom-speech about how some words are a bit vulgar and crass and we don't use these words. He agreed, and then informed me that sometimes, "Princess* uses that word. Then she giggles."
*not her real name :P