So it's clear that this is a question that we've answered, on account of having a 4 year old who will be a big sister soon.
The short answer came from living on a farm. The daddy and the mommy get together, and then the baby grows in mommy's tummy. The kid has seen this process from conception to birth, although I'm not sure it sunk in very much. I imagine that it will similarly be something she sees again, so that's the mechanics explanation taken care of.
In the people edition, we talked about how when a mommy and daddy love each other very much, sometimes they decide to have a baby, and the baby can grow inside the mommy.
I think that we'll probably talk about the additional complexities that go with this in a year or two, especially since adoption is potentially on the horizon. I think the best way for us to explain it is that sometimes mommies and daddies need help getting the baby to grow and sometimes that means some doctors need to help out. Sometimes other people help too by growing the baby in their tummy or helping in other ways that involve complicated science (and you can ask more about it when you are older, Kid).
In answer to the "how did the baby get in your tummy?" question, I'm at a loss. Go ask your other parent. We need to get to a sex ed class sometime soon so I feel more confident about how to have these conversations. Our faith community puts on a workshop series for parents and kids based on the Our Whole Lives curriculum so we'll have to find one to go to soon. Maybe I'll order the parent guide that goes with it now.
I think that I have no idea how to talk about adoption with the kid though, so I need to put that on my research radar for sooner rather than later, especially since we know some families with adopted children.
This is part of the PAIL monthly theme post series! Go check out what everyone else has to say!