Funny Mommy

Boy: (Pointing to the baby bathtub hanging in the upstairs shower): Why is this hanging here?
Me: That is where we keep the baby bathtub.
Boy: Why?
Me: So that we can use it if we need it.
Boy: Who would need it?
Me: A baby.
Boy: What baby?
Me: Any baby.
Boy: Like if a baby was at our house.
Me: Sure if a baby was at our house, his mom could use it for the baby.
Boy: But we aren't going to have a baby at our house are we?
Me: I don't know, maybe.
Me: Maybe it would be for a baby that we would have.
Boy: Us? (Sounding surprised) You're funny mommy.
Me: Don't you like babies?
Boy: Well sure... but.... you're funny mommy.

Like it would be totally funny to have another baby....
(No I'm not pregnant) But seriously... why does he think this is soooo funny?