The other night we were lying in bed talking (I have been trying to do this more with Lu, because I think she might have a lot on her mind, and it's about to get very hard to listen). She was yelling at my belly (to Lemon) and was insistent that she heard a response. I explained that Lemon can't make noise in there, because there's no air, only water, and that Lemon doesn't breathe, comparing it to an amphibian, which may have further confused things. Ultimately, I left at it the umbilical cord, more or less.
I said, "Everybody has a place on their body that shows where their umbilical cord was attached to their mommy. Can you guess what it is?"
"No, babe, their belly button."
Understanding, let alone explaining, the miraculous science project of pregnancy is a little beyond my pay grade, but I try.