A Cl-ass-ic Conversation

The other night I needed to run to the store for a few things. Tai was particularly tired, so I took the two younger kids (Perrin, 6, and Darcy, 4) with me. We piled into the car, and this is the conversation that ensued:

Perrin: Darcy, stop that or I'm gonna kick your ass!
Me: Whoa! What did you just say?!?
Perrin: I told Darcy to stop it or I'm gonna kick her ass!
Darcy: I'm gonna kick your ass, too!
Me: Nobody is gonna kick anybody's ass, and stop saying the word ass. It's not for little boys or girls!
Perrin: I can't say ass?!
Darcy: Why can't we say ass, Mommy?
Me: Stop saying ass! No, you cannot say ass, Perrin. Darcy, because I said so.
Perrin: What's ass mean, anyway?
Me: Ass means butt.
Darcy: If ass means butt, why can't we say ass?
Perrin: Yeah, why can't we say ass if ass means butt? *giggle*
Me: Because ass is a naughty word.
Perrin: But WHY is ass a naughty word?
Me: I don't know. It just is. Please stop saying ass.
Darcy: Ass means butt, right?
Me: Yes, Darcy. Stop saying ass!
Perrin: Ok, Mommy. We'll stop saying ass.
Darcy: Ok, Mommy. We won't say ass NO MORE!
Me: Thank you, guys.
**5 second pause**
Perrin: Darcy, stop that or I'm gonna kick your ass!