Author's Note
Happy Pi Day, everypony! I should have posted this yesterday, but I didn't even get the idea for it until past midnight, so I stayed up pretty late to finish writing it so that I could post it this morning. I hope you enjoy it, and that you had a great Pi Day of your own.
If you liked it, please do comment, because I like to know exactly what I did well so that I can keep it in mind for later stories. (Although I have nothing against likes and faves). If you didn't like it, that's fine, but again, please comment, because it's only fair. I want to know exactly what I did wrong.
Thank you. Enjoy. 
Pi Pie Pi
"Wait, run that by me one more time."
Twilight rolled her eyes at her clueless friend. "Rainbow Dash, I already told you. Weren't you listening?"
"Sure. Well, I tried. I can't help it if my brain automatically puts egghead comments on mute," Rainbow defended herself
The alicorn glared at her and huffed. "You are hopeless. Fine. I'll explain one more time. But that's it!" Clearing her throat in a businesslike manner, she began anew. "Today is Pi Day, a day to celebrate the mathematical constant pi, commonly referred to as twenty-two over seven. Pi Day is celebrated on March 14 every year, because March 14 can be written as 3/14 and the first three digits of pi are 3.14. Pi Day is celebrated throughout Equestria with free pizza, due to pi being a homonym of pie. And -"
"A homo-what?" Rainbow interjected, creasing her forehead in a puzzled frown.
"Never mind,' Twilight sighed. "Suffice it to say that I thought it would be a nice idea to invite all of my friends over for pizza." She gestured to the left, where Rarity and Fluttershy sat across from each other at a mahogany table set for six. Six neatly folded placards with an individual name on them as well as six plates were arranged symmetrically around the table.
"Well, that part I understood. But what's this 'pi' thingie?"
Twilight looked almost apoplectic with shock. "How can you never have heard of pi?!"
"Um, hello? I was training to become a Wonderbolt, not a rocket scientist."
Rarity sniffed and tossed her coiffed curls. "Well, I was training to be a clothes designer, and even I've heard of pi."
"Okay, if you're so smart, what is it?" Rainbow shot back.
The fashionista recoiled, taken aback. "Oh - well - it's - it's -"
"Thought so." The cyan pegasus crossed her hooves smugly and complacently looked away.
"Be quiet, Dashie. I know what it is. It's just hard to explain, that's all." Rarity floundered a moment longer, and then ventured, "It's what you use to calculate the area and perimeter of a circle... although I must say I've really never used it for anything else. Fluttershy? Perhaps you can enlighten our friend Dashie a little more?"
"Oh, um, no. I don't think so." Fluttershy bent her head, allowing her mane to slide over her face to conceal it from view as everypony turned to look in her direction. "I've never liked math much..."
Twilight shook her head pointedly. "Shameful."
"I'm sorry..." she whispered, mortified, cringing and sinking down until only the tip of her forehead could be seen above the rim of the table.
Rainbow Dash was still stuck on the idea of using pi to calculate area and perimeter. "But... how does it work? How can you find the area of a circle? It doesn't have sides!"
For once in her life, Twilight appeared stumped. "I - I don't know. That's a valid point..."
"And where the heck did the numbers 22 and 7 come from? Who even invented pi, anyway? And why did whoever it was have to name it after a dessert? 'Cause I totally got my hopes up when I heard 'pi day', only to discover it's not a day to celebrate dessert!"
These were valid questions (well, excluding the last one). Twilight reeled backwards, astonished. Never in her life had she heard so many questions that she didn't know the answers to asked all at once. "I - I really have no idea," she stammered. "I have no idea why pi works, I have no idea where it came from, I just use it in my calculations..." Her eyes were huge and frightened. "Sweet Celestia, I don't know!!!! How can I not know?! I have so many books related to mathematics! Why have I never read about the origins of pi?! This is bad! This is really, really bad! I need to go to the library! I need to -"
Rarity stood up and placed a soothing hoof on her friend's shoulder. "Calm down, darling! There's no need to fret. I'm sure the origins of pi can wait to be discovered a little longer. Why don't you allow yourself to rest for now, and refer to your library later? We're only waiting on Pinkie Pie, Applejack, and the pizza delivery pony, anyway."
Right on cue, the doorbell rang. Twilight took a huge gulping breath to calm herself down and smiled feebly at the alabaster unicorn. "Thanks, Rarity... yeah, I guess I can wait. I'd better go get the door."
"SURPRISEEEEEEEEEEEE!" a familiar, high-pitched voice sang, followed by a rush of sound as a pink blur zoomed through the front door.
"Hey, Pinkie," Rainbow greeted her, completely unfazed by her grand entrance. Such entrances were not at all out of the ordinary for Pinkie Pie, however out of the ordinary she might have been herself. "'Sup."
"A surprise, that's what!" Pinkie answered cheerfully, her eyes bright with mirth.
"I'm sorry, Pinkie, but there's no surprise," Twilight told her kindly. "I told everyone about this evening ages ago, so I fail to see -"
"No, not that surprise, silly! This surprise!" From nowhere, Pinkie produced five polka-dotted, cone-shaped party hats - which she somehow proceeded to jam on everypony's heads in five seconds flat - and a rain of confetti. "SURPRISE!!!!!!" Fluttershy shrieked as the confetti exploded into the air and dove under the table.
Rainbow spat a piece of confetti out of her mouth and grinned crookedly. "Twilight, there's always a surprise when Pinkie Pie's around."
Twilight looked frazzled. "Right... well... yes... Pinkie, why don't you sit down?" she offered lamely, trying to regain some of her dignity. "And... wait, wasn't my front door locked? How did you get in?"
"Okie dokie lokie!" the party pony chirped, completely ignoring her question, bouncing over to the table and giving Rarity a quick hug. "I can't wait for Applejack to get here so I can give her a hat!"
Dash smirked, picturing the look on AJ's face as Pinkie jumped at her with a polka-dotted hat and a sackload's worth of confetti. "Yeah..."
Fluttershy reappeared from under the table, trembling violently. "Is it s-safe to come out now?" she stuttered.
"Fear not," Rarity assured her gallantly. "The shenanigans are over. I think." She glanced over at Pinkie. "Right, darling? Pinkie?"
Pinkie was staring rather strangely - even for her - at her placard.
"Is something wrong?" Twilight asked worriedly, instantly forgetting her puzzlement over how Pinkie managed to come in through the front door despite its being locked.
Pinkie shook herself and smiled widely. "Oh, it's no biggie! My name is just misspelled on here, that's all!"
"What?" The alicorn peered closer at the name written in curling black script on the card. "No it's not. P-I-N-K-I-E P-I-E. It's fine."
"Oh, is that how you thought how my name was spelled?" Pinkie giggled. "Silly-willy! It's P-I-N-K-I-E P-I!"
A thick, torpid silence fell over the room.
It took Twilight Sparkle a good five minutes to find her voice again, and it even then it appeared that she had only partially recovered it. "W-w-wha-"
"What she's trying to say is, what in Equestria are you thinking?" Rainbow cut in. "We're not gonna fall for a joke like that. We're not stupid."
"But I'm not joking!" she protested.
"Darling, really," Rarity sighed. "Of course your name isn't spelled like that."
"It is! It so totally absolutely is! I'll show you!" Pinkie shot out of her chair and zoomed out of the front door, returning a few moments later with a sheet of paper that she had procured from goodness-knows-where. "See? It's my birth certificate!"
Oh. Apparently she had procured it from Ponyville Hospital. Which just happened to be on the other side of town. Then again, even the other side of town was only a few moments away if you were Pinkie Pie.
The other ponies huddled around the paper. Sure enough, there it was, in large letters right at the top: P-I-N-K-I-E P-I.
Dash was the first one to speak. "But..."
"I told you my name was spelled like that!" Pinkie stuck the paper somewhere into her mane, where it mysteriously vanished. "Now let's eat! Where's my pizza?" She looked around curiously, as though she hadn't just made everypony's minds implode.
"Pinkie..." Twilight's voice quivered. "Are you telling me that we've been spelling your name wrong ever since we met you?"
"Yep! Crazy, huh?"
"That's beyond crazy!!! That's - that's - that's ridiculous! And what were the odds of discovering that your last name was spelled 'P-I' on Pi Day itself?!"
Fortunately, the doorbell rang before Twilight could start calculating those odds. She shook herself out of her probability-induced reverie and trotted for the door. "That's the pizza delivery pony. Pinkie, we'll talk about your name later, when I have some paper and a pencil. I'm doing a paper about the origins of Equestrian surnames and I definitely want to interview you."
***
Half an hour later, all five ponies sat back in their seats, groaning and massaging their stomachs. "If I see another pie tonight - pizza or regular -I'm gonna throw up," Rainbow Dash groaned. Even Pinkie Pie, the pony who could eat ten cakes in one go - practically in the same mouthful - looked sick.
"Me too," Fluttershy agreed softly.
"I agree. I'm positive I weigh five pounds more," Rarity chimed in, glancing irritably at her slightly-protruding stomach. "I have no self-control when it comes to pizza, unfortunately."
Twilight barely even heard their conversation. "Where's Applejack?" she murmured. "I told her when to come. She's usually so punctual. She should have been here ages ago. I feel bad that we went ahead and ate without her."
"Don't feel bad, dear," Rarity consoled her. "The pizza would have gone cold. I'm sure Applejack will understand. In fact, she'll probably be along shortly."
"In the meantime," Rainbow interrupted, barely stifling a burp, "Let's make pie puns. In honor of Pi Day."
Everypony rolled their eyes. Rainbow Dash had recently become fond of delivering on-the-spot puns, and it was rather annoying.
"I'll start," she continued. "Okay. Got one. Which pony in this room likes Pi Day most?"
Nopony responded. They had learned to be wary of Rainbow's puns - they were usually horrible.
"C'mon!" Rainbow shouted. "Ask me!"
Twilight finally gave in with a sigh, if only to shut her up. "Fine. Which pony likes Pi Day most?"
"PINKIE PI!" Pinkie shrieked. "Am I right? Am I right?!"
"Uh.... I guess. But that's not the answer I was thinking of. Try again."
"Uh..." Pinkie thought for a while, and then shrugged. "Okay, I got nothing. Tell me! Tell me tell me tell me!!!"
A devilish gleam glinted in Dash's eye. "Pi-light Sparkle."
Twilight rolled her eyes. Rarity groaned and passed a hoof wearily over her forehead. "Kill me now."
Even Fluttershy, usually one of the first to offer a compliment, looked skeptical. Pinkie Pie, on the other hoof, clapped enthusiastically. "Good one, Dashie! Okay, my turn: Which dragon really likes pizza?"
"I'm going to go with... Spieke," Twilight responded dryly. "Spelled S-P-I-E-K-E."
Pinkie's whole body seemed to wilt. "Awww... how did you know?"
Rarity shuddered. "Please, no more pie puns. I feel sick enough without having to hear awful jokes about my dinner."
"Fine, no more pie puns."
"Thank you."
"I'll just say the word 'pi' a lot! It's so much fun to say!" Pinkie drew in a huge, gasping breath and then launched into her mono-word monologue. "Pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi -"
"Darling?"
"Pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi pi -"
"Pinkie, dear?"
Now she was modulating the tone of her voice. "Piiiiii PII piiiiiiiii PI piiiIIIiiiiiiiiIiiii -"
"Alright, she can't hear me. Or she's choosing not to listen." Rarity gave up and settled back in her chair, defeated. Fluttershy patted her back sympathetically.
Rainbow growled and clutched her ears as Pinkie's incessant pi's continued. "If I hear the word pi one more time tonight, I'm gonna scream!"
All of a sudden, the doorbell rang. Dash bolted to her feet, glad of an excuse to escape Pinkie Pie. "I'll get it!"
From the other side of the table, Twilight glared at her, her eyes popping and crazed. She had, apparently, had the same thought. "No, I will!"
After a brief tussle, which Rainbow totally won (because Twilight's mental muscles were definitely stronger then her physical ones), Dash shot over to the doorway, almost weeping tears of relief as Pinkie's voice faded into the distance. Hopefully, by the time Rainbow returned to the table, somepony would have managed to shut Pinkie up, and then Rainbow would never have to hear the word 'pi' again in her life.
She opened the door, smoothing down her polychromatic mane, which had been ruffled in her skirmish with Twilight. "Well, howdy there, R. D.!" a drawling country accent greeted her cheerfully. Applejack stepped forwards and gave Rainbow a tight one-hoofed hug, nuzzling the side of her neck. Then she stepped back, her emerald eyes crinkling as she smiled. "Sorry Ah'm so late. Ah wanted t' bring y'all somethin' special, t' thank y'all fer havin' me over." She brought up her other hoof, revealing a cardboard box from which emanated the scents of cinnamon and apples. Faint curls of steam rose from the top. "Ah baked yore fav'rite, Dash! A real, honest-t'-goodness Sweet Apple Acres apple p-"
Rainbow Dash slammed the door in her face.
