... Oh. Wow. I didn't even catch that reference. I never do. For some reason, my brain can't make the connection that Sirius and serious almost sounds the same. But I don't know why McGonagall would catch it. Doesn't she still think that Sirius is out to kill Holly? Or have I missed something? Why would she go "Oh, well, that's alright, then" when she finds out that Sirius sent it to her?