Neville gave Professor Lupin a look of pure terror, and Seamus Finnigan eyed the now rattling doorknob apprehensively.

"Boggarts like dark, enclosed spaces," said Professor Lupin. "Wardrobes, the gap beneath beds, the cupboards under sinks -- I've even met one that had lodged itself in a grandfather clock. This one moved in yesterday afternoon, and I asked the headmaster if the staff would leave it to give my third years some practice.

"So, the first question we must ask ourselves is, what is a boggart?" Hermione put up her hand as did Amelia.

"It's a shape-shifter," she said. "It can take the shape of whatever it thinks will frighten us most."

"Great." Amelia sighed. "I hate family reunions with wired relatives."

"Couldn't have put it better myself," said Professor Lupin, and Hermione glowed. "So the boggart sitting in the darkness within has not yet assumed a form. He does not yet know what will frighten the person on the other side of the door. Nobody knows what a boggart looks like when he is alone, but when I let him out, he will immediately become whatever each of us most fears."

"This means," said Professor Lupin, choosing to ignore Neville's 'mall sputter of terror, "that we have a huge advantage over the boggart before we begin. Have you spotted it, Harry?"

Trying to answer a question with Hermione next to him, bobbing up and down on the balls of her feet with her hand in the air, was very off-putting, but Harry had a go.

"Er -- because there are so many of us, it won't know what shape it should be?"

"Precisely," said Professor Lupin, and Hermione put her hand down, looking a little disappointed. "It's always best to have company when you're dealing with a boggart. He becomes confused. Which should he become, a headless corpse or a flesh-eating slug? I once saw a boggart make that very mistake -- tried to frighten two people at once and turned himself into half a slug. Not remotely frightening."