"So - straight into it. Curses. They come in many strengths and forms. Now, according to the Ministry of Magic, I'm supposed to teach you countercurses and leave it at that. I'm not supposed to show you what illegal Dark curses look like until you're in the sixth year. You're not supposed to be old enough to deal with it till then. But Professor Dumbledore's got a higher opinion of your nerves, he reckons you can cope, and I say, the sooner you know what you're up against, the better. How are you supposed to defend yourself against something you've never seen? A wizard who's about to put an illegal curse on you isn't going to tell you what he's about to do. He's not going to do it nice and polite to your face. You need to be prepared. You need to be alert and watchful."