I spent an hour or so on my own this afternoon with #2 Niece, while #1 was at school and my sister was out with #3 and #4. As she sat on my lap after I finished reading her a book, we had the following conversation:
#2 (a propos of nothing): When I grow up, I want to be a mummy.
Me: That's good. I'd like to be a mummy too.
#2 (turning to look seriously into my eyes): But you're not a mummy. Before you can be a mummy you have to get a baby in your tummy.
Me: I know.
#2: My mummy has had babies in her tummy. But you've never had a baby in your tummy.
#2: You can't be a mummy until you get a baby in your tummy, you know.
And so it went on. A 4-year-0ld has a great way of grabbing hold of a topic and not letting it go...