"They're getting hungry," said Lupin coolly, shutting his briefcase with a snap. "Dumbledore won't let them into the school, so their supply of human prey has dried up.... I don't think they could resist the large crowd around the Quidditch field. All that excitement ... emotions running high... it was their idea of a feast." 

"Azkaban must be terrible," Harry muttered. Lupin nodded grimly. 

"The fortress is set on a tiny island, way out to sea, but they don't need walls and water to keep the prisoners in, not when they're all trapped inside their own heads, incapable of a single cheery thought. Most of them go mad within weeks." 

"But Sirius Black escaped from them," Harry said slowly. "He got away..." 

Lupin's briefcase slipped from the desk; he had to stoop quickly to catch it. 

"Yes," he said, straightening up, "Black must have found a way to fight them. I wouldn't have believed it possible.... Dementors are supposed to drain a wizard of his powers if he is left with them too long...." 

"You made that dementor on the train back off," said Harry suddenly. 

"There are -- certain defenses one can use," said Lupin. "But there was only one dementor on the train. The more there are, the more difficult it becomes to resist."

 "What defenses?" said Harry at once. "Can you teach me?" 

"I don't pretend to be an expert at fighting dementors, Harry, quite the contrary..."

 "But if the dementors come to another Quidditch match, I need to be able to fight them --" 

Lupin looked into Harry's determined face, hesitated, then said, "Well... all right. I'll try and help. But it'll have to wait until next