A/N: Thanks for that, SylviaD, and I'll try to update as quickly as possible. Should be another chapter going up later today – the benefit of not going home for Christmas, I can write all day.

There's a fair bit of math-talk here, though I left out the actual equations. The paradox is a fairly classic one, you should be able to find more information simply by googling it.

Chapter Three: The boys talk, and Charlie poses a challenge.

-SE-

"So Don punched a guy out," Ian commented to Charlie once the others had all left to make the arrest. Charlie nodded.

"Yeah. It wasn't 'cause he teased me, though. I could ignore the teasing."

"Then what was it for?"

Charlie paused in cleaning the whiteboard, apparently considering the answer.

"Don and I were in the same year in high school," he eventually started again, not turning to look at Ian. "For the most part I kept out of his way – we didn't have any of the same classes, and moved in very different circles. He was one of the popular sporty guys."

"A jock," Ian commented, and Charlie winced, but nodded.

"Yeah, I guess. I mean, he was smart and all, but he wasn't as good at the academic stuff as I was."

"Was anyone?" Ian wondered aloud. Charlie bit back a laugh before continuing.

"I had Physics with one of Don's friends. Paul. And he tried to get me to do his homework for him, since he was Don's best friend. I gave him the same answer as everyone else who tried to bribe me into doing their homework: not gonna happen. He got all huffy, but didn't say anything more about it."

"And Don decked him for it?" He was feeling slightly uneasy as Charlie shook his head.

"Nah. I'd set up this experiment using the swings in the playground. It involved standing on the swings as they were moving. Paul threw a rock at me in the middle of the arc, which made me fall off. The rock and impact with the ground gave me a concussion, and a dislocated shoulder." Ian winced in sympathy, but Charlie just shrugged. "First thing Don did, as soon as the rock left Paul's hand, was deck him. By the time I hit the ground, he was next to me, making sure I was okay. Paul got expelled, and Donnie got suspended for a week. He told Paul that if he ever came near me again, it'd be the last thing he did."

"Wish my older brother had been like that," Ian let out before thinking. When Charlie turned to look at him in question, he mentally cursed himself.

"I didn't know you had a brother," the professor said in surprise. "What was he like?"

"Grade-A asshole," Ian admitted. "I was the middle kid, have a younger sister as well. We… don't talk much these days."

"How come?" Charlie sat down near him, and inside, Ian cheered. Maybe talking about family wasn't such a bad thing after all.

"Ah, she doesn't approve of my lifestyle. Thinks I should have become a normal teacher instead." Charlie tilted his head to one side, surveying the sniper, and grinned a little.

"I'm trying, but I really can't picture you in an indoors classroom. You certainly wouldn't fit in with any teacher I ever had."

"Yeah. I'm good at sniping and tracking though, so that's what I stick to these days."

"Good?" Charlie raised his eyebrows in indignation. "You're an amazing sniper!" He then bit his lower lip, apparently embarrassed at the outburst. Ian grinned at him.

"Glad you think so. Does that mean you'll be coming along to my beginner's course?"

"No chance in Hell," Charlie shook his head emphatically.

"Oh come on. You're a pretty good shot to begin with, you could be excellent with some training."

"Absolutely not. I fired a rifle twice. That's more than enough for me." He seemed adamant, and Ian backed off a little.

"We'll see," he murmured. "Well, I suppose I should go report to Merrick. See you round, Professor." With that he stood and left, not waiting for Charlie's farewell.

-SE-

"So what does a pre-Socratic Greek philosopher have to do with calculus?"

Agent Edgerton was sitting in the back of a Cal Sci lecture hall, arms folded across his chest, observing one of Charlie's lessons. One of the basics of sniping was to try to observe the prey in their natural habitat – and Charlie had never looked more natural. Sure he'd seen the professor lecturing the FBI agents, but that was only a peek at his teaching style. Here, he was in his element – free to go off on any tangent that crossed his remarkable mind, make number jokes that were actually understood, and he knew that all of his students actually wanted to be there listening to him.

"Well, Zeno was famous for his paradoxes. One of them was about Achilles and a tortoise. Basically, the problem is this: Achilles and a tortoise agree to a race, but the tortoise is unhappy because Achilles is very fast. So, the tortoise asks Achilles for a head start. Achilles agrees to give the tortoise a 1,000m head start. Does Achilles overtake the tortoise? The problem has also been described by someone trying to shoot a moving tortoise with an arrow, and figuring out how far the tortoise would travel before the arrow hit them. Both problems basically equate to the same thing – how to work out the relative distances covered by two moving objects.

"Now using differential calculus, we can solve this in one of two ways. The obvious way, is to start by writing…" And he was off, writing symbols that Ian was barely able to follow. It had been a long time since high-school calculus, but he was enjoying watching the lesson. All around him, Ian saw students taking notes and copying the formulae from the board.

"This, however, is not the method that Zeno used to approach his own paradox. Instead, he broke it down like this…" And Charlie started on the other whiteboard, in a much more complicated equation. It took a while to explain to everyone, but Ian could vaguely understand it.

"So, what's the upshot of all this? Basically, Zeno figured that you could shoot at as many tortoises as you like, because as long as the little buggers are moving, you can't actually catch them." There was a ripple of quiet laughter, and even Ian let himself smile. Charlie then glanced at his watch and heaved a dramatic sigh.

"Alas, I see we have nearly exceeded our allotted time for this week. I want you all to do the exercises in the text book to make sure you understand, and I'll see you all same time next week."

The students all packed up and filed out, though a few went to the front of the hall to discuss something with their professor. Ian waited for the crowd to thin a little before getting up and slowly making his way down as well. He caught a few curious glances from the students still there, but most didn't seem fazed by his presence. After all, Professor Eppes often had people sitting in on his lectures, and sometimes even brought unexpected guests in to talk about various real-world applications for mathematics.

Charlie himself, however, trailed off in shock as he saw the sniper. He'd thought that the older man would look out of place in an academic setting, but he seemed to be able to fit into any environment with little difficulty.

Ian reached the front of the class as Charlie got over his surprise and grinned at him.

"Agent Edgerton! I didn't expect to see you here."

"Yeah, well, if you're not going to visit my classroom, I figured I'd come see yours."

"And did you learn anything?"

"Apparently you want me to go shoot tortoises."

"No, no!" Charlie held up his hands and tried to back-pedal, whilst the students still hanging around giggled.

"Yes, I quite clearly heard you," Ian couldn't stop the grin from forming. "You were encouraging us all to go shoot some defenceless reptiles. Since when were you such a sadist?"

"I give up," Charlie hung his head, still smiling.

"Good. Does that mean you'll come take shooting lessons?" Ian saw the surprise on the students' faces, but ignored them.

"Like I said before, Edgerton," Charlie shook his head, "there's no chance in Hell that you'll get me firing a rifle again."