"Severus," said the headmaster, very seriously, "I want you to promise that you will not tell anyone – anyone – about what you have seen this evening."
"But, Professor! Remus Lupin's a werewolf! He's dangerous!"
"I am well aware that he is a werewolf," said Dumbledore calmly. "I knew it before he ever arrived at Hogwarts. Do you really believe I am so ignorant of what goes on in my own school, Severus?"
"But people have a right to know!" Snape argued. "It isn't fair to everyone here to put them in that kind of danger without them knowing –"
"Be quiet," said Dumbledore, in such a tone that Snape momentarily fell silent, chastened.
"Do not make the mistake of thinking that I would put the safety of any student here at risk. Remus is taken to that shack, through the passage Sirius told you about, well before moonrise on every full moon. The building is warded so that he does not escape and nobody can come across him by accident while he is transformed. Had you not insisted on wandering the grounds after dark and prying into other people's business, you would have been perfectly safe."
Snape's mouth opened. He could not believe Dumbledore was telling him not to mess around with spying and breaking curfew! What about Potter and his gang? But before he could speak, Dumbledore held up a hand. The headmaster had not finished talking. His eyes pierced Snape's, and Snape felt pinned to the spot, unable to look away from that bright blue gaze.
"Listen to me, Severus. Remus has found happiness here at Hogwarts, just as you have. Like you, he had a childhood that was less than pleasant, through no fault of his own..." Snape stared back at the headmaster, speechless. How had he known –
"By giving away Remus's secret, you would be taking something very precious away from him," Dumbledore went on. "He would be forced to leave this school. And so would you, if you harmed him like that."
Snape was stunned. "You wouldn't!" he gasped.
"I most certainly would," said Dumbledore, sternly. "You must understand something right now, Severus. It is a terrible thing to give away another person's secrets. If everybody knew Remus Lupin was a werewolf, his life would be made miserable. It is his affliction, so he is the one who chooses whether or not to tell anybody. Sirius should not have said anything to you about it. I have had a word with him already."
"What about Potter?" asked Snape. "He and Black nearly got me killed."
"From Remus and Sirius's accounts, it sounds to me as though James saved your life," said the headmaster. Snape frowned. He knew it was true, and there was no lying to someone like Dumbledore. He said nothing.
"You will all stay in the hospital wing overnight," said Dumbledore, "and you can go to class tomorrow morning as usual. But, Severus: you are not going to tell anybody about what you saw, are you? Because if you do, I will have to expel you from Hogwarts."
"No," whispered Snape, with a sidelong glance at Potter, who lay asleep several beds away. "I won't tell anyone. If they find out, it won't be from me."