Puzzled but interested, the class got to its feet and followed Professor Lupin out of the classroom. He led them along the deserted corridor and around a corner, where the first thing they saw was Peeves the Poltergeist, who was floating upside down in midair and stuffing the nearest keyhole with chewing gum.
Harry was still an underage wizard, and he was forbidden by wizard law to do magic outside school. His record wasn't exactly clean either. Only last summer he'd gotten an official warning that had stated quite clearly that if the Ministry got wind of any more magic in Privet Drive, Harry would face expulsion from Hogwarts.