Couldn't this wait until you reach the age of 3, at least?

A few days ago Rebecca was laying down for bed and reached up and took my face in her hands. "Mama?" she asked me. "My baby sister Madeline was in your tummy yesterday?" (She's got the concept that yesterday means before this moment, and tomorrow is in the future, but the actual concept of time, not so much)

I explained that Madeline was in my tummy a few months ago, but now she's out and growing up to be a big girl, just like Rebecca. She paused for a moment then said "You just... squished her right out!"
She's remarkably close on that one.

Then last night we were doing the bedtime routine again, and once again she took my face in her hands and said "Mama? How'd Madeline get in your tummy"

Seriously? Could we have the birds and bees talk when you're a bit more mature. Like 3? I paused for a few seconds, trying desperately to think of how I wanted to answer this question. I don't want to lie to her or mislead her, but she obviously doesn't need to know any kind of detail. So I finally settled on "Mommies and Daddies can sometimes make babies and they go in Mommy's tummy". Not exactly what I wanted to say, in hindsight, but it works.

How in the world do you explain this sort of thing to a 2 year old?