"Are you familiar with the Patronus Charm, Severus?"
"The Patronus Charm?" Snape shook his head. "I've heard of it, but never tried casting it. There are other ways of dealing with dementors."
"This is the best way that does not involve Dark magic," said Dumbledore, in a neutral tone. Snape's eyes narrowed, but he did not try to defend himself against the implied rebuke. "Watch."
Dumbledore pulled back his sleeves, said, "Expecto patronum!" in a clear voice, and a shining silver phoenix blossomed from the end of his wand. The small instruments on the shelves around the walls of the office glittered in its light. The apparition hovered for a moment in front of him, and then vanished. Snape glanced at Fawkes, who was sitting on his perch as usual.
"You see, it is an external manifestation of the things a dementor consumes: happiness, hope, the will to survive ... but it is impossible to harm it, because the Patronus cannot feel the fear and hopelessness created by dementors. It's a useful protection against them; when you conjure one successfully, it shields you and drives dementors back. And every Patronus takes a different form: what it looks like depends on the wizard who conjures it. Sometimes," Dumbledore added, half to himself, "a Patronus reflects some quality in ourselves that we never even realized. Some wizards have said that the image of a Patronus expresses one's innermost heart.
"But the Patronus is useful in another way," he continued, smiling. "After quite a lot of experimenting, I have found a method of using Patronuses as messengers. Since they are immune to Dark magic and cannot be disguised or faked – and since conjuring the Patronus is a rather difficult piece of magic to begin with – they make a nearly perfect form of communicator. That is why I want you to learn this spell. In the Order of the Phoenix, we use Patronuses to send short, urgent missives.
"Now, you remember the words..."
"Yes, yes, I heard you. Expecto patronum," said Snape, a little impatiently. His wand was already out. He squared his shoulders, cleared his throat, and said: "Expecto patronum!"
"Oh, I almost forgot the most important part," said Dumbledore. "You need to concentrate very strongly on a happy memory in order to produce a Patronus."
There was a pause. Dumbledore watched Snape's face curiously.
"Is that so..." said the younger wizard. "Well. Let me think about that for a minute."
"Certainly ... take as long as you need ..." murmured Dumbledore, as Snape's gaze became unfocused, looking within. Dumbledore sat down at his desk, picked up the Daily Prophet and pretended to be busy reading it, as Snape searched his mind for a happy memory.
The sound of Snape's voice crying: "Expecto patronum!" again made him look up. What he saw made him drop the newspaper in surprise.
A beautiful, bright silver doe had appeared in front of Snape. Dumbledore knew where he had seen a Patronus like that one before, though it had been many years ago. It was exactly the same shape as Lily Potter's. Snape stared at his spellwork with his mouth open, and as he blinked, it vanished.
Snape whirled on Dumbledore, with a mixture of shock and anger on his face. "Don't – don't say anything! I –"
Dumbledore said nothing. Snape sat down rather suddenly, covering his eyes with his hand. His shoulders shook a little. Dumbledore watched him, but he remained quite silent. For a long time, he trembled but did not move.
After several minutes, Dumbledore ventured: "As I said, the form a Patronus takes is unique to the wizard conjuring it. It often looks like something we were not expecting..."
Snape lowered his hand, and looked Dumbledore in the face. He was as pale as parchment, but his eyes blazed. "You will not mention this to anyone! You wanted me to learn this spell, and so I have, but do not ask me to use it!"
Dumbledore leaned back in his chair. "Yes, it seems you've learned it. That was remarkably fast, but then you are quite a skilled wizard, Severus." Snape's expression did not soften.
Dumbledore coughed. "Ahem. I did promise you that I would not mention the reasons behind our ... agreement, to anyone. Ever. I still plan to keep my word." His gaze did not waver.
Snape let out a breath he seemed to have been holding. He looked away. "Fine."
"You may go," said Dumbledore. And Snape was gone.