McGonagall: I need to leave early.
Sprout: Detention with Potter and Black again?
McGonagall: *sighs* Yes. Thank Merlin for Lupin's good influence; without him I wouldn't have any free time at all.
-Meanwhile in the Gryffindor common room-
Prongs: So you're saying that there's a spell that will turn the Slytherin robes red and green?
Padfoot: Yes! I was thinking tomorrow after dinner-
Moony: Absolutely not!
Padfoot: Aw, come on Moony! It's harmless.
Moony: You will not be turning the Slytherin robes red and green.
Moony: Not when there's a spell that will make them see-through.