"Ah, I see," he nodded, happy with the first half of the explanation. That explained why she seemed new but not-quite-new, and possibly why she looked vaguely familiar as well.
But the next part threw him. "You're my what?" he asked, but his brain caught up almost as soon as the words were out his mouth. Oh, shit-
"You're joking..." he said, although he had the distinct feeling that she wasn't.