I've gone mad. That's the only explanation. I've finally cracked under the stress of it all.

"Ten AM, Thursday." Snape's voice comes from behind me now, not in front, while I'm bent over, hands on my knees.

"Eleven AM, Thursday." This version of Snape's voice comes from in front of me.

I want to look up, but I'm worried I'll barf again. I can feel Snape's hands clamp down firmly on my shoulders and he leads me back, away. But at the same time, I can see Snape in front of me, walking forwards to close the door.

"Clean that up," I hear him say to somebody, before the door shuts, and seals off all noise.

Snape leads me back to the correct classroom. I'm still dizzy, and slump gratefully into my seat. A small pill and a glass of water are placed on my desk.

"Anti-nausea," Snape says. "Take it. Now."

I do so, with shaking hands.

"Did she…?"

"Yes, Theo," Snape snarls. "She did." He glares daggers at me. "We're going to have to begin locking doors. I can see having you in the class will be much like supervising a toddler, Greengrass. Now, can you tell me what that was?" he asks.

"Motion sickness?" I guess.

"How very profound."

I think for a moment, but I don't want to say it. Not out loud. I'll sound mad. I feel mad.

"Miss Greengrass is giving us a lovely demonstration of fragmented disorientation," Snape tells the others. "Let this be a reminder to you all. No activity, not even college football," this he aims at Malfoy, "involving the use of your time-turner is permitted without my express permission and supervision. Thankfully, this threat is contained to Miss Greengrass alone. Now, I will ask you again. Can you tell me what that was?"

"Us," I whisper. "That was… us."

"Correct." I doubt Snape's tone could ever soften, but it smooths out, at least. "That is us, an hour from now."

I gulp. "That's impossible."

"Not with that golden chain around your neck, it's not." Snape draws himself to full height. "Time-turners make the impossible, possible. Their very names should be explanation enough. You will be using it to take multiple classes in the same hour. Had you not so recklessly decided to go stumbling about," his voice turns icy, "you would have had days, if not weeks, to acclimatise to the idea before learning that multiple versions of you are in existence with each use. We avoid contact with other versions of ourselves as much as possible. Most students of mine go their entire time here at Hogwarts without ever making that error." Snape glowers, not just at me, but the entire class. "It would seem you are all determined to break that rule."

A student with circular glasses yawns, stretching his arms out. "Are we going to begin, Professor? It's just that the rest of us have all been using time-turners for weeks now, and it's probably a bit soon for a refresher."

"Watch your tongue, Pucey," Snape warns. But he strides across to the blackboard. "I trust you all have your completed essays analysing Dante's Inferno."

"Yes," I blurt out, too hasty in excitement.

Snape raises an eyebrow. "And what did you decipher from the text? A simple interpretation of morality, perhaps? A lightbulb moment of connecting the symbolism, in a way you think so very profound but the rest of us touched on within the first ten minutes of the class?"

"No," I blush. "I chose to discuss Dante's political perspective and overtones."

"Is Dante's Inferno meant to be political?" somebody whispers.

"Shut up, Theo," Pansy hisses.

"Please," Snape says. "Enlighten us."

I pull my parchment from my satchel and glance over it quickly. I'm glad to have something to do, to focus on, besides what else had happened this morning.

"I think there's a disdain for politicians, and to an extent, politics as a whole. I think it goes without saying the text is religious in nature, and there's several hints throughout that politics and matters of state are in direct contradiction of religion as a whole. That, if left unchecked, they might surpass religion entirely. Dante's Inferno is a revenge piece. Against the state." My eyes flicker to the others, their faces dropped in surprise. Draco even looks, dare I say, impressed. I'm suddenly embarrassed. "I wrote a bit more about it, if you'd like to read," I mumble.

Snape swoops down and takes my parchment. He does not criticise or otherwise put me down. I suppose, coming from Snape, that must be a sign of approval.

After handing in our essays, Snape moves onto Shakespeare's narrative composition. I take eager notes, wishing I'd had a class like this before the dreadful essay I handed into McGonagall. Despite his intimidating manner, when teaching, Snape manages to not only explain things in a profound way, but somehow elicit equally profound thoughts from us. Even Gregory Goyle, who I learn has trouble spelling his own name, spoke for more than a minute about Hamlet's atypical plot structure being reflective of the titular character's mental state. The hour flies by far faster than I'm used to, and it's only when Theo points out the time my anxiety returns.

"Close your eyes," Draco mutters beside me, as we walk to the next classroom. "It'll help with the nausea."

I eye him suspiciously. "You're being awfully kind today."

"Don't count on it lasting," he mutters, but his voice lacks its usual edge.

I'm familiar with the classroom, noting the spot where I was sick has been scrubbed clean. I take my seat and, on Snape's command, retrieve my time-turner. I turn the hour back once more.

I'm on edge for the beginning of my first architecture lesson, waiting for the moment I come bursting through the door. Sure enough, before Snape's even finished drafting the blueprint, the door flies open. I see myself bent over, vomiting on the floor. I thought I'd been prepared for this, but it's still uncanny. Snape doesn't even glance at me — me, me — because he's too busy dealing with past me. And then the other Snape arrives, and my head starts to spin again. The whole thing has to be a paradox. I have so many questions.

"Clean it up," Snape tells me, as he closes the door.

I should have known my future self, now my present self, would be the one to clean it. I'm no stranger to hard work, and so I fill a bucket with soapy water and get to scrubbing.

"Professor," I say, when I can bear the questions in my head no longer. "What would happen if we changed things? Say I didn't clean up, or we'd decided to go to a different classroom after I saw this one. Would it make a difference?"

"It would make all the difference in the world," he replies, still sketching on the blackboard as the others copy it on parchment. "It is one of the most fundamental reasons why we do not interact with our other selves. To do so, and to deliberately alter the events that occur, would cause a rip in the timeline. Can anybody give an example of such a rip?"

"Louis Howley," says Theo, "In nineteen fifty five. He re-visited his lunch hour, hoping to get some extra study in, and saw his girlfriend breaking up with his future self. He avoided her for the rest of that day, and the breakup never happened."

"And what were the consequences of his meddling?" Snape asks.

It was Draco who spoke next. "He created a rip in the timeline. Nobody knows exactly what chain of events occurred, but Louis wrote himself out of existence. He disappeared. The only evidence he ever existed at all were people's memories of him."

That sounds like Vincent Crabbe, I think. I'm sure to scrub extra hard after that, terrified even a tiny speck of vomit could lead to the same fate for me.