Depends. I've gotten a wide range of answers, but "Why twelve?" is pretty common, which sometimes segues into a whole discussion about whether "twelve" is or isn't a legitimate move in the conversational turn-taking game, which allows me to raise the question of whether "Why?" is. (Of course, depending on the age of my interlocutor, this exchange takes different superficial forms.)

But, sure, sometimes it just brings the exchange to a crashing halt.