A/N: Prologue set in season 4.
Disclaimer: I don't own BTVS.
Willow Rosenberg sat, chin in hand, staring towards the front of the room in fascination. Her notes lay on the desk in front of her, derivatives and integrals completely forgotten, as she hung on to every word her Calculus teacher said.
"Um, Mr. Griffin?" a male voice somewhere behind her interrupted. Startled, Willow turned around to face him as he said, "Class is over."
In disbelief, Mr. Griffin, Willow, and several other students looked to the clock.
"Damn," the teacher sighed, flipping through his notes. He was a young man, not too long out of school himself, and really passionate about math. So passionate, in fact, that sometimes he forgot he was supposed to be teaching out of a calculus textbook, and went on and on about interpretations of infinity, or the huge size of Graham's number, or, on one particularly memorable occasion, perceptions of reality.
("This is red," he stated one day pretty early in the year, holding up a folder from his desk. "But is the red I see the same red you see? When you look at the sky, is 'your' blue the same as 'my' blue? Don't we always describe colors in relation to other colors? So, how could we figure out if we were seeing the same colors?" He paused to let that idea soak in before continuing. "Does English sound the same to my ears when I speak it as it does to yours? When I look at Maddie's face" – he gestured to a student in the front row – "do I see the same Maddie as everyone else in this room? Or does she look different to each of us? And how would we know?"
His students gaped, some laughing in disbelief at the idea, others holding their heads in their hands in confusion at the possibility that what he was saying was true. Twenty minutes later, when they left the class, almost all of them were still pondering the thought.)
Today, the off-topic concept had been even more confusing than the one about reality and perception, and had taken up over half of the class. There were several pages of notes remaining in Mr. Griffin's hands, and a moment later he set them down, accepting defeat. "Okay, we're going to have to finish this tomorrow. Got off topic again," he chuckled. "Just push the homework on your syllabus back one day. See you all Thursday."
Willow, as usual when Mr. Griffin went off on these discussions, left the class with a goofy grin on her face, the nerd in her loving the confused, overwhelmed feeling as her brain tried to process concepts too big for its design. The math-loving grin, however, was soon replaced by an equally goofy Tara-loving grin, as she saw her girlfriend waiting for her outside the classroom.
"Hi, baby," she greeted her lover with a small kiss, wrapping her arms firmly around the taller girl's waist. "Good day?"
Tara nodded with a shy smile. "B-better now. I believe you said something about t-taking me on a date tonight?"
"I guess I can squeeze you in," Willow shrugged. She laced her fingers through Tara's as they began to walk off. All thoughts of Mr. Griffin, calculus, and infinity were forgotten in the presence of her lover.
Several hours later, as they were getting ready for bed in Tara's dorm room, the thoughts came back to her. She slid into bed, waiting for Tara to join her, still marveling at the ideas Mr. Griffin had presented to them earlier that day.
"What are you thinking?" Tara's voice whispered at her ear.
Willow jumped. Deep in thought as she was, she hadn't even noticed her girlfriend get into bed next to her. "Math stuff. I mean, calculus is always overwhelming, but Mr. Griffin said some stuff today that seriously blew my mind."
Tara's eyes were closed with her cheek resting on the redhead's shoulder, but Willow knew she was listening intently. "What did he say?"
Smiling at the chance to explain, she launched right in. "He was talking about these alternate universes. Like, there are an infinite number of universes existing in this same space, parallel to each other, where each tiny little instant, each of them splits into infinitely more universes. And it just keeps going on and on, and they're right here but we can't touch them or breach the gap between them."
"Um, sorry, but huh?"
Willow paused for a breath. "Okay. So Mr. Griffin explained it like, if there's a die and you're rolling it every infinitely tiny part of every little second. And, oh, well, the die has to have infinity sides. And whatever side it lands on represents one possibility of something that could happen in that universe."
"Okay."
"But that's not the only universe that exists. Because, at that instant that you roll the die, the universe splits up, and for each possibility on the die, you have a new universe. And then you roll the die in those universes and make more new universes. So he said crazy stuff, like, there's a universe where his head caught on fire today. Or normal stuff, like one where we actually finished going over the lecture, so I'm doing homework right now."
"That makes my brain hurt," Tara complained, nuzzling into Willow's shoulder.
"Me too," the other girl admitted, "but I kind of like it. It's so cool! Plus, I already sort of knew about this stuff. There's a universe where I'm a dominatrix-slash-vampire. And also, still kinda gay."
Tara blinked. "Again, um, huh?"
"Long story," Willow brushed it off, "and also not as much fun as you might think. I felt violated by myself. Which was strangely hot, but mostly just strange."
"Hmm," the blonde sighed, laughing a little. "I think I get it."
"Yeah?" Willow smiled. "Isn't it cool?"
"Very." One side of her mouth pulled up in the lopsided smile that made Willow melt a little. "So, for example, there's a universe where I'm not about to kiss you right now."
"Sure hope it's not this one."
"Not a chance," she assured her lover, and then kissed her and reached for the light.