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

Harry cast his mind about for a happy memory. Certainly, nothing that had happened to him at the Dursleys' was going to do. Finally, he settled on the moment when he had first ridden a broomstick. 

"Right," he said, trying to recall as exactly as possible the wonderful, soaring sensation of his stomach.

 "The incantation is this --" Lupin cleared his throat. "Expecto patronum!" 

"Expecto patronum, " Harry repeated under his breath, "expecto patronum." 

"Expecto patronum," he could hear Amelia mutter to herself. 

"Concentrating hard on your happy memory?"

 "Oh -- yeah --" said Harry, quickly forcing his thoughts back to that first broom ride. "Expecto patrono -- no, patronum -- sorry -- expecto patronum, expecto patronum" 

Something whooshed suddenly out of the end of his wand; it looked like a wisp of silvery gas. 

"Did you see that?" said Harry excitedly. "Something happened!" 

"Very good," said Lupin, smiling. "Right, then -- ready to try it on a dementor?" 

"Yes," Harry said, gripping his wand very tightly, and moving into the middle of the deserted classroom. He tried to keep his mind on flying, but something else kept intruding.... Any second now, he might hear his mother again... but he shouldn't think that, or he would hear her again, and he didn't want to... or did he? 

Lupin grasped the lid of the packing case and pulled. 

A dementor rose slowly from the box, its hooded face turned toward Harry, one glistening, scabbed hand gripping its cloak. The lamps around