Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

She'd been Head of House Gryffindor for long enough, she'd watched enough friends die, to know that there were some people you couldn't save from becoming heroes.

There came a knock at the door, and Professor McGonagall said, "Enter."

When Harry entered, his face had the same cold, alert look she'd seen in Mary's Place; and she wondered for an instant if he'd been wearing that same mask, that same self, this whole day.

The young boy seated himself on the chair before her desk, and said, "So is it time for me to be told what's going on?" Neutral the words, not the sharpness that should have gone with the expression.

Professor McGonagall's eyes rose in surprise before she could stop them, and she said, "The Headmaster told you nothing, Mr. Potter?"

The boy shook his head. "Only that he'd received a warning that I might be in danger, but I was safe now."

Minerva was having trouble meeting his gaze. How could they do this to him, how could they lay this upon an eleven-year-old boy, this war, this destiny, this prophecy... and they didn't even trust him...

She forced herself to look at Harry directly, and saw that his green eyes were calm as they rested on her.

"Professor McGonagall?" the boy said quietly.

"Mr. Potter," said Professor McGonagall, "I'm afraid it is not my place to explain, but if after this the Headmaster still does not tell you anything, you may come back to me and I will go yell at him for you."

The boy's eyes widened, something of the real Harry showing through the crack before the cool mask was set back in place.

"In any case," Professor McGonagall said briskly. "I'm sorry for the inconvenience, Mr. Potter, but I need to ask you to use your Time-Turner to go back six hours to three o'clock, and give the following message to Professor Flitwick: Silver on the tree. Ask the Professor to note down the time at which you gave him that message. Afterward the Headmaster wishes to meet with you at your convenience."

There was a pause.

Then the boy said, "I am suspected of misusing my Time-Turner, then?"

"Not by me! " Professor McGonagall said hastily. "I am sorry for the inconvenience, Mr. Potter."

There was another pause, and then the young boy shrugged. "It'll play hob with my sleep schedule but I suppose it can't be helped. Please let the house elves know that if I ask for an early breakfast at, say, three A.M. tomorrow morning, I'm to receive it."

"Of course, Mr. Potter," she said. "Thank you for understanding."

The boy rose up from his chair and gave her a formal nod, then slipped out the door with his hand already going under his shirt to where his Time-Turner waited; and she almost called out Harry! only she wouldn't have known what to say after.

Instead she waited, her eyes on the clock.

How long did she need to wait for Harry Potter to go back in time?

She didn't need to wait at all, actually; if he had done it, then it had already happened...

Minerva knew, then, that she was delaying because she was nervous, and the realization saddened her. Mischief, yes, unspeakable unthinkable mischief with all the prudence and foresight of a falling rock - she didn't know how the boy had tricked the Hat into not Sorting him to Gryffindor where he obviously belonged - but nothing dark or harmful, not ever. Beneath that mischief his goodness ran as deep and as true as the Weasley twins', though not even the Cruciatus Curse could have gotten her to say that out loud.

" Expecto Patronum ," she said, and then, "Go to Professor Flitwick, and bear back his reply after you ask him this: 'Did Mr. Potter give you a message from me, what was that message, and when did you receive it?'"

One hour earlier, having used the last remaining spin of his Time-Turner after putting on the Cloak of Invisibility, Harry tucked the hourglass back into his shirt.

And he set out toward the Slytherin dungeons, striding as quickly as his invisible legs could manage, though not running. Thankfully the Deputy Headmistress's office was already on a lower floor of Hogwarts...

A few staircases later, taken two steps but not three steps at once, Harry stopped at a corridor around whose final bend lay the entrance to the Slytherin dorms.

Harry took a piece of parchment (not paper) out of his parch, took a Quotes Quill (not pen) out of his pouch, and told the quill, "Write these letters exactly as I say them: Z-P-G-B-S-Y, space, F-V-Y-I-R-E-B-A-G-U-R-G-E-R-R."

There were two kinds of codes in cryptography, codes that stopped your little brother from reading your message and codes that stopped major governments from reading your message, and this was the first kind of code, but it was better than nothing. In theory, no one should read it anyway; but even if they did, they wouldn't remember anything interesting unless they did cryptography first.

Harry then put that parchment in a parchment envelope, and with his wand melted a little green wax to seal it.

In principle, of course, Harry could've done all that hours earlier, but somehow waiting until after he heard the message from Professor McGonagall's own lips seemed less like Messing With Time.

Harry then put that envelope inside another envelope, which already contained another sheet of paper with other instructions, and five silver Sickles.

He closed that envelope (which already had a name written on the outside), sealed it with more green wax, and pressed a final Sickle into that seal.

Then Harry put that envelope into the very last envelope on which was written in large letters the name "Merry Tavington".

And Harry peeked around the bend to where the scowling portrait that served as the door to the Slytherin dorms waited; and as he did not wish the portrait to recall not-seeing anyone invisible, Harry used the Hover Charm to float the envelope to the scowling man, and tap it against him.

The scowling man looked down at the envelope, peering at it through a monocle, and sighed, and turned around to face toward the inside of the Slytherin dorms, and called, "Message for Merry Tavington!"

The envelope was then allowed to fall to the floor.

A few moments later the portrait door opened, and Merry snatched up the envelope from the floor.

She would open it up and find a Sickle and an envelope addressed to a fourth-year student named Margaret Bulstrode.

(Slytherins did this sort of thing all the time, and a Sickle definitely constituted a rush order.)

Margaret would open her envelope, and find five Sickles along with an envelope to be dropped off in an unused classroom...

... after she used her Time-Turner to go back five hours...

...whereupon she would find another five Sickles waiting for her, if she got there quickly.

And an invisible Harry Potter would be waiting in that classroom from three PM to three-thirty, just in case someone tried the obvious test.

Well, it had been obvious to Professor Quirrell, anyway.

It had also been obvious to Professor Quirrell that (a) Margaret Bulstrode had a Time-Turner and (b) she wasn't very strict about how she used it, e.g. telling her younger sister really good pieces of gossip "before" anyone else had heard.

Some of the tension leaked off Harry as he strode away from the portrait door, still invisible. Somehow his mind had still managed to worry about the plan, even knowing that it had already succeeded. Now there remained only the confrontation with Dumbledore, and then he was done for the day... he'd go to the Headmaster's gargoyles at 9PM, since doing it at 8PM would seem more suspicious. This way he could claim that he'd just misunderstood what Professor McGonagall had meant by "afterward"...

The obscure pain clutched at Harry's heart again as he thought of Professor McGonagall.