Disclaimer: This story does not contain explicit sex, so if that's what you're looking for, this may not be your cup of tea.
Thanks goes to Stella_Omega and Rozezwild for their advice and editing.
Somehow, he had anticipated that knock on the open door. She had gotten into the habit of showing up in his classroom after hours irregularly a long time ago, whenever she could make time to see him, which wasn't often. But somehow, he had felt that this would be one of those days, and his entire body felt lighter in an odd way when he heard the knock.
He smiled, not even looking up from the homework he was grading.
"Hello, Sadie."
"Am I that predictable?" she inquired, and his smile grew wider as he raised his eyes and allowed himself a long look over the pale, brunette girl that was standing in the doorway, leaning her head against the wall to one side. Sadie was small, but elegant, with high cheekbones and expressive brown eyes that he couldn't stop himself from gazing into for a few seconds. Her slender arms were wrapped around her folder, books and notebook, just like she had always entered his class when she has still been in high school. Her medium brown hair, always completely straight, never pinned up or pulled back, fell an inch or so below her shoulders.
He marveled at how little she had changed in the last few years. She had lost the timid look in her eyes a little bit, college seemed to agree with her in that regard. Her lips also weren't quite as bitten as they used to be, and she had finally gotten rid of the huge baggy sweaters she had used to hide in for so long, and wore a t-shirt that was actually in her size.
"I just had a hunch," he said finally, and Sadie came closer, almost shyly. Things were always awkward between them at first, before they got used to talking as equals again, not as the student and teacher they had used to be. She gave him a small smile and pulled up one of the student chairs with a charring noise that made him grimace. Most of the rubber feet on the chairs in his classroom were broken or gone, a fact he hated, but couldn't change.
He watched as Sadie took a seat. Her small frame still fit into the chair perfectly, just like it had more than four years ago, when she had been in his class. She was the only thirteen-year-old he had ever had in his Calculus class, and he had been fascinated with her and the way her mind worked from the very start. Sadie had breezed through Calculus, but stayed after class often to discuss advanced problems or ideas she'd had with him. He had started to challenge her soon, with everything from tricky integrals to theoretical questions that required a high level of abstract thinking. She rose to each challenge magnificently. Sometimes she would storm into his classroom first thing in the morning, with flushed cheeks and gleaming eyes, to present a solution to him. She had been the most enthusiastic student he had ever taught, by a far stretch.
And she still had her passion, he noticed when she put down the stack in her arms. Atop lay a Linear Algebra textbook, which was pleasure reading for Sadie by now.
"I'll wait if you want to finish this," she offered, flicking her lovely eyes to the homework in front of him. He shook his head and pushed the stack away, focusing his attention on her as she played with the part of her bookmark that stuck out of the book. She looked so young just sitting there that he could hardly believe that she was in her third year of college now. She had had a birthday a month ago, but he wasn't going to congratulate her if she didn't bring it up. It wouldn't feel right admitting that he knew her birth date from the top of his head.
He leaned back in his chair, swivelling it sightly. "How are your classes?" he asked, watching as her face broke into a smile right away.
"Mostly they're really fun, except for Astronomy. I hate having to get up in the middle of the night."
"For Astronomy? Imagine that," he teased her, enjoying her innocently flushed cheeks.
"Differential Equations is my favorite," she changed the subject. "It's easy. I've been looking ahead in the book, mostly at systems and Laplace Transforms, although I don't have a complete grasp of them yet."
"The transforms are difficult to grasp," he agreed. "Especially once you get around to transforming square waves and all kinds of other abominations."
That made her laugh. "Did you have trouble with those when you were in college?"
He looked at her, rubbing his temples. "You're making me feel old, you know. I don't even remember."
"You're not old," she said. Her cheeks flushed slightly as she looked up at him, her eyes darting over his face briefly. "You're not old," she repeated. He gave her a brief, sincere smile and changed the topic.
"How are the other aspects of college? The food?"
"Still horrible," she said without hesitation, making him laugh out loud.
"The people?"
She shrugged. "Nice. My professors are really helpful outside class. I enjoy that."
"How are you getting along... socially, I mean?" he asked her, and the expression on her face changed at once. He hadn't meant to cause her any pain by asking the question, but she had had problems even in high school, and he had been the person she had ended up confiding in.
"It's... it works out," she said reluctantly. He could read in her face that she didn't like lying to him, but she did anyway, maybe out of embarrassment, because she had trouble understanding the people around her. He had noticed a long time ago that she had the social awkwardness that troubled many highly gifted students. Not wanting to be the cause of any more of her discomfort, he changed the topic once more.
"I just remembered, I believe I have a Differential Equations book here that gives examples of some of the more complicated Laplace Transforms. You know, the composites." He stood up from his chair and walked past Sadie to one of the bookshelves where he kept his personal collection of advanced math books. He grabbed a dark green one without hesitating and turned back to the desk, flipping the book open and flying through the pages while standing. Sadie was next to him, very close, watching him flip through the book with interest. Finally, he found what he was looking for.
"Here," he pointed, pushing the book to the side so she could read it over his shoulder. Sadie leaned forward, her hair brushing over his shoulder, and when he took a breath, he could smell her intoxicating scent. She was wearing a bit of perfume, he noted with some surprise.
"That's amazing," Sadie commented on the table he was showing her. "I'm curious about where some of that comes from, I may have to calculate some of the integrals just for fun."
"You would," he laughed.
