It might be the question that expectant parents of large families hear most often (when maybe child number 4, certainly child 5, 6, 7, ... is on the way).
The first 112 times you just laugh it off. At some point you want to say, "Do you think that's clever?" or "Do you think I haven't heard that one before?" but refrain because you don't really want to start something, and most people don't know what to say and try to get by by attempting humor.
By the time number 8 rolls around, you do hear a lot more "Congratulations," I guess because people do eventually realize that you seriously think children are a blessing and not something you're trying to avoid. For those who still insist on asking the question, your patience is about at an end and you have a visceral need to respond differently, if for no other reason than there seems to be a biological limit to pretending to laugh at something that was never that funny to begin with. Clearly, charity suffers; here is an incomplete list of things you think, but still rarely say.
Umm. [chuckling along]. I'm sorry, how what happens?
As a matter of fact, we do. But I don't know how to get my wife to stop dreaming about baby unicorns.
Oh, do you need some pointers? Go ahead, ask anything. What are friends/relatives for?
Honestly? No, we don't. Perhaps you could explain it to me.
[awkward silence as you let the question just hang there]
No, I wasn't paying attention in health class. I was too busy sneaking peaks at the Catechism.
Well, we thought about having a nicer house and a lake place and new cars, but decided it just wasn't worth it.