How do you get a baby in your belly?

I suppose I knew that the question would eventually come up. Of course, I have always said that I am ready for this question. In fact, I have given many others advice about how to handle this inquiry. And, I have not thought twice about passing judgment on those who did not handle it the right way. So, last night when my four year old asked, "How do you get a baby in your belly?", one might have wished that my house was set up to video tape how I expertly handle this kind of moment.
I choked. I completely choked. My response was something very close to the following, "Oh well, mommies and daddies do some stuff and then a baby gets in the mommy's belly." I didn't make eye contact and acted generally dismissive as I was perusing the Pottery Barn website for towel bars when I was rudely interrupted with this ridiculous question.
My son's response was to request that me and daddy "do some stuff" and "put a baby" in my belly. To this I responded, "ok".
My plan of action going forward is general avoidance and guilt. I think I am really breaking ground here and blazing a trail for mommies everywhere.