"What's that?" said Harry. 

"Another boggart," said Lupin, stripping off his cloak. "I've been combing the castle ever since Tuesday, and very luckily, I found this one lurking inside Mr. Filch's filing cabinet. It's the nearest we'll get to a real dementor. The boggart will turn into a dementor when he sees you, so we'll be able to practice on him. I can store him in my office when we're not using him; there's a cupboard under my desk he'll like." 

"Okay," said Harry, trying to sound as though he wasn't apprehensive at all and merely glad that Lupin had found such a good substitute for a real dementor.

 "So..." Professor Lupin had taken out his own wand, and indicated that Harry should do the same. "The spell I am going to try and teach you is highly advanced magic, Harry -- well beyond ordinary Wizarding Level. It is called the Patronus Charm." 

"How does it work?" said Harry nervously. 

"Well, when it works correctly, It conjures up a Patronus," said Lupin, "which is a kind of anti- dementor -- a guardian that acts as a shield between you and the dementor." 

Harry had a sudden vision of himself crouching behind a Hagridsized figure holding a large club. Professor Lupin continued, "The Patronus is a kind of positive force, a projection of the very things that the dementor feeds upon -- hope, happiness, the desire to survive -- but it cannot feel despair, as real humans can, so the dementors can't hurt it. But I must warn you, Harry, that the charm might be too advanced for you. Many qualified wizards have difficulty with it." 

"What does a Patronus look like?" said Harry curiously. 

"Each one is unique to the wizard who conjures it." 

"And how do you conjure it?"