"So, not a science teacher, then."

It's out before he can stop it. For a startled second he just watches her face, trying to determine whether he ought to apologize. On the one hand, the way she talks about her mum indicates she probably wants and needs a sympathetic ally, but on the other hand he's not sure how rude it is to casually (albeit mildly) insult the mother of your first and hopefully only partner.