The bright lines danced about in the silence for a moment before Harry wet his lips. "What sort of mistake?"
"Oh, all the little should-have-beens," Dumbledore said. "Chance encounters that never happened, plates of chips never ordered, illnesses recovered from an hour early. Inconsequential things that ought to have happened, and didn't."
"But how does that work?" Neville asked. "How can there be a mistake if things just...happen?"
"Oh, Time has a fairly good idea of where things are going," Dumbledore said with a kindly smile. "Think of it as water poured onto a slope. There is only one direction it can go, and if you study the slope you can usually predict which path the water will take. This is how the Seers practice their art; they can See, in part, how the water of Time is going to flow. But place a pebble in the right juncture and you can change how the water flows. And if a bird flies over and drops a twig, things can be diverted in completely unpredictable ways." Dumbledore raised his wand and tapped the point where Harry and Neville's timelines first intersected, turning it red again. "This point here, for example, represents something unexpected happening that changed how events turned out. In most cases, Time is simply diverted in another direction, and the flow continues toward the intended result. But sometimes - not often, but rarely - a small event diverts things just enough that it causes other, more serious diversions, for years on end - perhaps centuries - to the point where there are so many diversions from the original flow that it cannot fill the spaces. We, quite literally, run out of Time, and life cannot survive if it does not have Time to survive within." A chill ran down Harry's spine, remembering the red bleeding off into nothingness.
"So this device can tell the future?" Neville asked. Dumbledore chuckled.
"Not usually, no - the timelines are never this clear, and I imagine it is drawing upon the Time still inherent in your memories to create something this complete. No, what this device can do is show a general idea of where the current is likely to flow. It is why I was convinced Voldemort would return, and why I am reasonably certain he shall fall - the paths that are most likely to lead to his defeat are paths that will likely be followed. I assume that your presence here, yours and Harry's, means that he will indeed be vanquished, as he must be." He smiled a little sadly as he contemplated a line. "Ah, I see that I am not long for this world," he said as he tapped a line that ended abruptly with his wand. Harry opened his mouth and Dumbledore held up a hand in abeyance. "I suspected it, and it is good to know for certain - it means that I can make some necessary preparations." He turned to Harry, a certain pain showing on his face. "You understand, my boy, why I had to do the certain things I did - and will continue to do?"
Harry nodded. "I didn't at first. But I did at...at the end."
Dumbledore nodded. "I am not proud of myself for putting you in such a situation, but I am remarkably proud of you for doing what has to be done." He turned back to the timelines, continuing as though he had not interrupted himself at all. "This explosion in the Department of Mysteries, when they were attempting to create new Time-Turners - your guess was quite apt - is what happens when Time unravels itself when there are too many diversions from what must happen. They attempted to draw upon stores of Time that were no longer connected to any possible future. You were caught in the backlash - had possibly even been manipulated by Time to be there in the right place at that moment. Time has an unusual intelligence of its own."
"For what purpose?" Harry asked, his gaze returning to the small red point.
"Why, Harry, I would think you'd be used to it by now," Dumbledore said with a small smile. "You must save the world." And he tapped a second silver clockwork device.
Emerald green smoke floated into the air around the diamond bright lines, winding about them like spun sugar before snapping into place as lines of their own, above and slightly behind the white lines like a shadow.
"This is one possible way small events could be changed to prevent the dangerous diversions that sent you here," Dumbledore said. "It is one of many, but the first is usually the one closest to how time actually ended up."
Harry leaned closer to the red pinpoint of light highlighting the first intersection of his and Neville's lines. "It's slightly different here," he said, reaching out with one finger. "In the real timeline, Ron's line intersects mine before Neville's. But in the green one, Neville's is first."
Neville had paled. "Harry," he said, "I was going to open the door to your compartment on the train, but then Trevor hopped out of my hands and I had to go chase him. Ron ended up opening the doors instead, and I couldn't find Trevor..."
"An astute observation," Dumbledore commended. "You see how one tiny, inconsequential event can change the entire path of one's life. Had you opened the door first, on that first train to Hogwarts, you and he would likely have shared the close friendship that ended up being the province of Harry and Ron. That is not to say," he continued as Harry's face fell slightly, "that Ron would not have been your friend, nor that Miss Granger would not have joined your coterie. As you can see, if you follow the lines, it would appear that the four of you will embark upon the many adventures that were reserved for just you three the first time around." He smiled slightly. "It rather reminds me of another foursome that roamed the halls of this castle, led by another young man by the name of Potter." Harry smiled slightly.
"So we're supposed to change history?" Neville asked in surprise.
"I daresay you've already changed it," Dumbledore pointed out. "But when you change history is every bit as important as how."
Harry stared hard at the green timelines as they twisted their way across the room. "But we were sent to this time," he protested. "How are we supposed to...fix things, or whatever we're supposed to do, if we weren't sent to the correct time?"
"Oh, I imagine you were both sent to the exact point you needed to be," Dumbledore said, eyes twinkling. "You see, this particular device has been out of order until very recently - this very afternoon, in fact."
"And I...um. I'm sorry, Professor, but I break that device in a few months," Harry said, both excitedly and apologetically.
"Oh?" Dumbledore said with a raised eyebrow, looking amused. "And under what circumstances does this happen?"
Harry fidgeted. "I probably shouldn't say, and it's not...very interesting...anyway," he said finally.
"I see," Dumbledore said, nodding slowly. "At any rate, you were sent here to obtain the vital information and understanding it has imparted to you, as well as learn of the mission upon which it would appear you have been sent. I imagine that Time shall provide both the opportunity and the means to do what it needs doing in order to put into motion a future that can happen." He waved his wand as though collecting cobwebs, the white and green timelines both winding about it, then deposited the timelines into a flask, which he corked and handed to Harry. "What needs to be done should be fairly obvious at the time," he said as Harry inspected the flask, "But should you need to consult the cheat sheet, you have it there. Unstoppering the flask should release the point in time you are currently inhabiting." He smiled gently. "And do not be dismayed if it seems you cannot recall things. Your reactions to events must be genuine, not calculated, and being able to remember your lives as you know them will disrupt that."
Harry stowed the flask away safely in his school bag, his brain churning with questions. "If everything is supposed to be genuine and unplanned, why send us here in the first place for an explanation? Why not just send us back to fix things without a clue?"
"Because a choice needs to be made," Dumbledore said seriously. "And only you two can decide if you are going to make it. You are undoubtedly going to be sacrificing a great deal of the lives you know in order to ensure that this new timeline comes to pass. You will have to choose whether the sacrifice is worth it."
"Didn't you say that if history doesn't get changed, the entire world ceases to exist?" Neville asked.
"Something to that effect, yes. Although it won't cease to exist; it will just cease to continue. There is a subtle, but important, difference."
"So basically we have to choose between our lives as we know them and saving the world," Harry said sarcastically. "That's a hard decision."
Dumbledore did not smile. "Harder than you can probably comprehend. Keep in mind that ceasing to continue will not cause anguish or pain to anyone. Everything will simply...stop. Changing the timeline so that life can continue will mean that life will continue - along with all the pain and suffering and joy and love that life has to offer."
"Still not something that will take much thinking," Harry said.
Dumbledore bowed his head. "I would not expect you to make any other decision, when life is on the line," he said somberly. "Also keep in mind that you shall retain memory of what your life once was, alongside what your new life will be; you will not be allowed to be oblivious."
"What if we choose wrong?" Harry asked. "If we're not remembering what we're supposed to be doing..." he trailed off as Dumbledore shook his head.
"The right decisions will stem very naturally from who you are and what you believe to be right," he said very seriously. "That is likely why Time chose you to be its tools."
"How is it that you know so much about what is going to happen?" Neville asked suddenly.
Dumbledore winked. "Suffice to say that this is not, to use the common phrase, my first rodeo." A diamond bright light suddenly shone from the doorway behind them, and Neville and Harry turned in surprise. "If I may say so," Dumbledore said, his voice sounding as though it was coming from very far away, "Happy traveling."