He's about to go back to do a little more cleaning as Rita turns to leave, but her question makes Strange stop, pausing before he picks up some of the papers on the table.

"It is," Strange answers. "The Ringmaster made it." And then, with a curious frown, he can't help but ask, "How did you know?"

Because Rita's not the only person who's somehow known more about magic than they realized. After all, Strange was able to tell that Childermass was doing some sort of magic near him a few evenings ago, without even knowing what the magic was.