We have five (in seven years). When I was expecting my 5th, I was in the grocery store. The cashier's eyes got wide when she saw my four little ones and realized I was having another. She said, "Don't you know what causes that?"
I replied, very cooly, "Yes, and my husband and I enjoy it so much, we can't stop!"
Well, she got so red in the face and embarrassed, that I decided that was my response to all those thoughtless clods!!
I think I would have said, "Yes, but I'm not sure what causes rudeness."
Great response!