Is that an asymptote in your pocket

Making her way downtown, trotting fast, Rarity was Golden Oaks-bound. A saddlebag bounced at her side, reminding her with every thump of the heavy book inside it. 

She knew she should have said “no, thank you” when Twilight insisted she took it home to read. She knew she would hate it—really, who would like a risqué romance book titled Calculust except for Twilight?—but she’d been too polite to say no and so now there she was. Returning it that same day, the word “tangenitals” seared into the depths of her mind, ready to haunt her every waking hour until she was dead and then probably beyond that. 

“Tangenitals,” she whispered, still horrified. “My stars.”

If Twilight asked her what was wrong with it, she would simply say that the prose wasn’t really captivating her, simple as that. Hopefully she wouldn’t needle Rarity on what specifically it was, because Twilight really loved the book and who was Rarity to eviscerate her apparently abominable taste in novels? 

The pace of her step increased once she saw Golden Oaks in the distance. Soon, she would be rid of the weight of romance writing so mediocre it might have put her off the genre a little bit. 

…Alright, maybe she was being a tad uncharitable. It wasn’t that awful. It had helped her appreciate her good books all the more. 

A sign hung from the library’s doorknob, stating the library was closed at the moment. Of course, this only applied to regular Ponyvillians, and not Twilight’s friends. With that in mind, Rarity opened the door and barged in, initially singing out Twilight’s name only for her to stop mid-sentence and choke on her words at the sight before her. 

There was no sugarcoating it. No trying to hide it, or pretend she hadn’t seen, or, frankly, going back to normal life. What she saw, she could not unsee. 

It being Rainbow Dash and Twilight Sparkle—who were not dating—standing in the middle of the lobby, making out.   

Obviously, they stopped as soon as Rarity barged in, the unicorn and pegasus practically jumping away from each other in flustered horror. 

“Rarity!” they yelped. 

Rarity had many a thought racing in her mind, so she grabbed the simplest one and voiced it: “Why were you two making out in the foyer?”

“We weren’t. You’re dreaming,” Rainbow Dash replied immediately, which earned a groan from Twilight. 

“Rarity,” she said, as harrowed as she was red, which was as a lobster. “I can explain, I—”

“Why the foyer?” Rarity interrupted. Bear in mind, this was not her only question, but it was the easiest one to get to without needing a glass of wine. “Who picks the foyer of their house to make out?”

Rainbow snorted. “Uh… I mean. Why not? Any place is a hot place to make out, right?” she said, turning to Twilight in search of confirmation, but instead Twilight was intently studying the floor. She did, however, slap a hoof against her forehead when Rainbow gasped and exclaimed, “Not that we, like, were making out or anything!”

“Rarity!” Twilight managed, directing her frustration towards the innocent unicorn. “The library is closed! Did you not see the sign?!”

Rarity scoffed. “Twilight, what are you talking about? That’s for normal ponies, not your friends!”

Twilight slammed her hoof on ground. “It’s for EVERYPONY!”

“Wait, really?” Rainbow asked.

“Sweet Celestia!” Twilight exclaimed, doubtless examining her life choices, Rarity thought. “Holy—”

“Stop!” Rarity interrupted again. “Everypony stop! I need to process this.”

While Twilight and Rainbow stood there, awkwardly waiting for Rarity to… do something, the unicorn closed her eyes, rubbed her forehead and processed. 

“How long has this been going on?” she finally asked, eyes still closed. 

“…A month,” they replied. 

A month! That wasn’t too horrific. Any longer than that and Rarity would have been appalled they’d kept her in the dark. 

“A month,” she repeated, finally looking up at her friends and then asking, “And why, pray tell, has this been a secret?”

It wasn’t like she or the other girls would have disapproved. Certainly, Rarity had thoughts on the dynamics of such a pair, but she would have never been anything but supportive. So, then, why? 

There was a moment of pause, followed by both mares hanging their head and saying, perfectly in unison: 

“Because it’s embarrassing.”

This was promptly followed by both of them going wide-eyed in shock, turning to each other and indignantly exclaiming: “What?! You’re embarrassed to be dating me?!”

“You’re an egghead!” Rainbow exclaimed, as if this was an obvious justification. “You’re a nerd! You do smart stuff that’s, like, not COOL, Twilight!”

Twilight arched an eyebrow, cold. “Oh. So the things I like aren’t cool?”

“T-that’s not what I meant!” Rainbow stammered, backtracking. “I think you’re cool! It’s just, like, other cool people don’t think you’re cool!”

“Oh? Well, same for you!” Twilight protested. “You’re a jock! My peers in the academic world don’t think you can even—”

“Well!” Rarity interrupted before things got worse. “Clearly, there are some unresolved matters here which I would love to help with once I fetch my wine. But, first—” Finally, she fished the book out of her saddlebag and levitated over to Twilight. “Here. What I actually came for.”

Before Twilight could even react, it was Rainbow Dash who spoke: 

“Wait, is that Calculust?!” Rainbow exclaimed. “I love that book!”

“You what?” asked the two unicorns, saying the same thing while sporting expressions from complete opposite sides of the spectrum of emotions. Twilight looked surprised, enchanted, delighted, and quite flustered, too. But Rarity? Horror, disappointment, shame, horror again. 

“Yeah, yeah!” Rainbow continued, grabbing the book and leafing through it, ignoring her two friends re-examining so many things in very different ways. She then snorted and said with a mischievous grin, “Man, it even has my favorite word ever.”

Rarity tried to stop Twilight from asking. She knew the answer from the moment she saw Rainbow’s infantile grin. She tried to stop it, but Twilight was faster.

“What word?” she asked, forcing Rarity to sit there and hear it yet again. 

“Tangenitals, haha!”

But that wasn’t the worst part, oh no. No, what truly made that moment one that would curse Rarity forever was Twilight Sparkle blinking and then giggling affectionately. Endeared. Charmed. 

“It is really funny,” she admitted. 

Again, many a thought shot through Rarity’s brain, but the first and foremost was a sudden clarity—an understanding, if you will—of why these two mares were compatible as they both giggled together lovingly.  

They were both idiots.