Deconstructing Mary's Lamb

Tonight, as I sang Tot-Tot (age not-quite-3) his special-request song, "Mary Had A Little Lamb"...the following conversation occurred:

Me: (singing) "...he followed her to school one day, which was against the rule..."
TT: (interrupting) Why was it against the rule?
Me: Tot-Tot, I am singing.
TT: But why? Why was it against the rule?
Me: Because you can't bring an animal to school.
TT: But I bring my animals to school.
Me: Yours are stuffed. You can't bring a real animal to school. Like our cat, you can't bring her to school.
TT: Why can't I bring our cat to school?
Me: Because she is alive.
TT: Why is she alive?
Me: Because she breathes and eats and makes a pee-pee.
TT: (like this is news to him) How does she make a pee-pee?!?!
Me: (avoiding the actual question) She goes in the litter box.
TT: And sometimes she vomits. And that is NAUGHTY.
Me: Yes, sometimes she vomits and I don't like that.
TT: But why did Mary bring a lamb to school?
Me: Well, actually, the lamb followed her, so she didn't really bring it.
TT: So, the lamb was naughty?
Me: It's time for night-night.