"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 Hagrid-sized 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?"

"With an incantation, which will work only if you are concentrating, with all your might, on a single, very happy memory. "

Chapter 12