Bad Words

The kids were playing pleasantly together in Noah's room while I was in the kitchen, and I heard them having a discussion about bad words. Here's how it went:
Noah: Why can't we say whatever words we want to say?
Sarah: I don't know.
Noah: Go ask Mom.
Sarah (coming into the kitchen): Mom, why can't we say whatever words we want?
Me: Because some words are not good for you to say.
Sarah goes back to Noah's room and says: Because some words are not good to say.
N: Which ones?
S: I'll ask Mom....Mom, which ones?
M: Well I'm not going to say them because that would require saying them.
N: We've heard you say them before anyway.
M: Well, I'm trying not to say them.
N: Can we say butt?
M: No.
N: Stupid?
M: No.
N: Shut up?
M: No.
N: [unrepeatable expletive]?
M: Certainly not.
N: But you say it when you're mad.
M: Yes, well...I shouldn't.
N: Why can't we say it? What's so bad about it?
M: It hurts people's feelings and makes God sad.
Several minutes of relative quiet, and then Sarah comes out again, sent on another mission.
S: Can we just say the bad words quietly to ourselves?
M: No. Because if you say them in your mind, they will eventually come out of your mouth.
She reports this to Noah, and he laments, "Oh, man. I really want to say bad words!"