How Do Babies Grow in Mummy's Tummy?

Here it comes, the big question: How did babies come about?

I told her that we kissed, and then she started forming. Then she grew and grew, so large my stomach couldn't contain her, so she came out.

She was confused, of course, and asked: "Did you kiss many times, and keep kissing? And then I started growing inside you?"

I gave an answer that was misleading but received as a child: "Yes. Must kiss until you came." And I tried to avoid more questions by changing the topic.

I was caught off guard and was not ready to explain in details. But this question tells me it's time I start preparing a decent and logical answer for her soon.