"I'm sorry," Harry said. He felt wretched at this point, he'd just told off Gandalf essentially, and Dumbledore's kindness was only making him feel worse. "I shouldn't have distrusted you."
"Alas, Harry, in this world..." The old wizard shook his head. "I cannot even say you were unwise. You did not know me. And in truth there are some at Hogwarts who you would do well not to trust. Perhaps even some you call friends."
Harry swallowed. That sounded rather ominous. "Like who?"
Dumbledore stood up from his chair, and began examining one of his instruments, a dial with eight hands of varying length.
After a few moments, the old wizard spoke again. "He probably seems to you quite charming," said Dumbledore. "Polite - to you at least. Well-spoken, maybe even admiring. Always ready with a helping hand, a favour, a word of advice -"
"Oh, Draco Malfoy!" Harry said, feeling rather relieved that it wasn't Hermione or something. "Oh no, no no no, you've got it all wrong, he's not turning me, I'm turning him."
Dumbledore froze where he was peering at the dial. "You're what?"
"I'm going to turn Draco Malfoy from the Dark Side," Harry said. "You know, make him a good guy."
Dumbledore straightened and turned to Harry. He was wearing one of the most astonished expressions Harry had ever seen on anyone, let alone someone with a long silver beard. "Are you certain," said the old wizard after a moment, "that he is ready to be redeemed? I fear that whatever goodness you think you see within him is only wishful thinking - or worse, a lure, a bait -"
"Er, not likely," Harry said. "I mean if he's trying to disguise himself as a good guy he's incredibly bad at it. This isn't a question of Draco coming up to me and being all charming and me deciding that he must have a hidden core of goodness deep down. I selected him for redemption specifically because he's the heir to House Malfoy and if you had to pick one person to redeem, it would obviously be him."
Dumbledore's left eye twitched. "You intend to sow seeds of love and kindness in Draco Malfoy's heart because you expect Malfoy's heir to prove valuable to you?"
"Not just to me!" Harry said indignantly. "To all of magical Britain, if this works out! And he'll have a happier and mentally healthier life himself! Look, I don't have enough time to turn everyone away from the Dark Side and I've got to ask where the Light can gain the most advantage the fastest -"
Dumbledore started laughing. Laughing a lot harder than Harry would expect, almost howling. It seemed positively undignified. An ancient and powerful wizard ought to chuckle in deep booming tones, not laugh so hard he was gasping for breath. Harry had once literally fallen out of his chair while watching the Marx Brothers movie Duck Soup, and that was how hard Dumbledore was laughing now.
"It's not that funny," Harry said after a while. He was starting to worry about Dumbledore's sanity again.
Dumbledore got himself under control again with a visible effort. "Ah, Harry, one symptom of the disease called wisdom is that you begin laughing at things that no one else thinks is funny, because when you're wise, Harry, you start getting the jokes!" The old wizard wiped tears away from his eyes. "Ah, me. Ah, me. Oft evil will shall evil mar indeed, in very deed."
Harry's brain took a moment to place the familiar words... "Hey, that's a Tolkien quote! Gandalf says that!"
"Theoden, actually," said Dumbledore.
"You're Muggleborn?" Harry said in shock.
"I'm afraid not," said Dumbledore, smiling again. "I was born seventy years before that book was published, dear child. But it seems that my Muggleborn students tend to think alike in certain ways. I have accumulated no fewer than twenty copies of The Lord of the Rings and three sets of Tolkien's entire collected works, and I treasure every one of them." Dumbledore drew his wand and held it up and struck a pose. "You cannot pass! How does that look?"
"Ah," Harry said in something approaching complete brain shutdown, "I think you're missing a Balrog." And the pink pyjamas and squashed mushroom hat were not helping in the slightest.
"I see." Dumbledore sighed and glumly sheathed the wand in his belt. "I fear there have been precious few Balrogs in my life of late. Nowadays it's all meetings of the Wizengamot where I must try desperately to prevent any work from getting done, and formal dinners where foreign politicians compete to see who can be the most obstinate fool. And being mysterious at people, knowing things I have no way of knowing, making cryptic statements which can only be understood in hindsight, and all the other small ways in which powerful wizards amuse themselves after they have left the part of the pattern that allows them to be heroes. Speaking of which, Harry, I have a certain something to give you, something which belonged to your father."
"You do?" said Harry. "Gosh, who would have figured."
"Yes indeed," said Dumbledore. "I suppose it is a little predictable, isn't it?" His face turned solemn. "Nonetheless..."
Dumbledore went back to his desk and sat down, pulling out one of the drawers as he did so. He reached in using both arms, and, straining slightly, pulled a rather large and heavy-looking object out of the drawer, which he then deposited on his oaken desk with a huge thunk.
"This," Dumbledore said, "was your father's rock."
Harry stared at it. It was light gray, discolored, irregularly shaped, sharp-edged, and very much a plain old ordinary large rock. Dumbledore had deposited it so that it rested on the widest available cross-section, but it still wobbled unstably on his desk.
Harry looked up. "This is a joke, right?"
"It is not," said Dumbledore, shaking his head and looking very serious. "I took this from the ruins of James and Lily's home in Godric's Hollow, where also I found you; and I have kept it from then until now, against the day when I could give it to you."
In the mixture of hypotheses that served as Harry's model of the world, Dumbledore's insanity was rapidly rising in probability. But there was still a substantial amount of probability allocated to other alternatives... "Um, is it a magical rock?"
"Not so far as I know," said Dumbledore. "But I advise you with the greatest possible stringency to keep it close about your person at all times."
All right. Dumbledore was probably insane but if he wasn't... well, it would be just too embarrassing to get in trouble from ignoring the advice of the inscrutable old wizard. That had to be like #4 on the list of the Top 100 Obvious Failure Modes.
Harry stepped forward and put his hands on the rock, trying to find some angle from which to lift it without cutting himself. "I'll put it in my pouch, then."
Dumbledore frowned. "That may not be close enough to your person. And what if your mokeskin pouch is lost, or stolen?"
"You think I should just carry a big rock everywhere I go?"
Dumbledore gave Harry a serious look. "That might prove wise."
"Ah..." Harry said. It looked rather heavy. "I'd think the other students would tend to ask me questions about that."
"Tell them I ordered you to do it," said Dumbledore. "No one will question that, since they all think I'm insane." His face was still perfectly serious.
"Er, to be honest if you go around ordering your students to carry large rocks I can kind of see why people would think that."
"Ah, Harry," said Dumbledore. The old wizard gestured, a sweep of one hand that seemed to take in all the mysterious instruments around the room. "When we are young we believe that we know everything, and so we believe that if we see no explanation for something, then no explanation exists. When we are older we realise that the whole universe works by a rhythm and a reason, even if we ourselves do not know it. It is only our own ignorance which appears to us as insanity."