Circumference Over Diameter
When Twilight first told Pinkie about it, the pink mare didn't seem very convinced.
"Pie Day? But Bake A Pie Day was last Tuesday! Or are you talking about Throw A Pie Day? Now that's a whole different shebang."
"No, not Pie Day, Pinkie, Pi Day!"
"Isn't that what I said?"
"No, pi! P, I, pi!" Twilight spelt. "It's a mathematical constant! Surely you must've heard about it before!"
"A mathematical constant that has pee in its eye? That doesn't sound very hygienic."
It was at this moment that Twilight began to regret talking to Pinkie about mathematical matters. This was almost as bad as the time the hyperactive party planner had attempted to multiply the area of her mouth by her height, of course, to calculate the maximum volume of her speech. Or last month, when she was feeling a little down, and Twilight had told her to look at things from the right angle — resulting in Pinkie sitting by herself in the corner of the room for almost five hours.
But today, sitting under the fresh, spring sun and the cool, rustling breeze, sipping icy chocolate milkshakes at their favourite park bench in Ponyville Park, Twilight decided she could be a bit more patient for the time being.
"Pi, Pinkie. It's the value you get after dividing the circumference of a circle by its diameter."
"I had a circumference once," Pinkie offered, "But it had to be surgically removed."
Twilight shoved her face into her hoof. "No, no, Pinkie, a circumference is the length that goes around something! Here, this might help."
She squeezed her eyes shut and summoned a pie and a long piece of string into existence. They landed with a silent thud onto the wooden table, the fragrant aroma emanating from the pie tantalising their nostrils in tangy temptation.
"See, I told you it was a pie!" Pinkie grinned, finally understanding, a steady stream of drool beginning to drip from her muzzle.
"No, no, no, forget about it being a pie!" The alicorn groaned, hastily sliding it away from Pinkie, who was about to swallow it whole. "My point is, the top of the pie is round. It's a circle."
"Mhm!"
"And if you take this string, and wrap it around the circle," She paused as she demonstrated, Pinkie watching intently. "Then you know the length of the string that goes around it, and that is the circumference of the circle!"
"Gotcha!"
"And the diameter," Twilight adjusted the string. "Goes across the circle, passing through the middle, of course, to get the longest length."
"But I thought that was the circumference?"
Twilight looked up from the pie and stared at Pinkie. "Excuse me?"
"You said the length of the string is the circumference. So how come the length of the string is also the diameter?" Pinkie cocked her head to one side in contemplation. "I really don't get it."
Twilight wanted more than anything to hurl the pie into Pinkie's face. However, she resisted the urge, not because it wouldn't be right for the Princess of Friendship to do so, but because it would be a huge waste of a perfectly good freshly-baked cherry pie.
"Let's try a different approach," she decided, gritting her teeth as she set the string aside. "Got any measuring tape?"
"You betcha!" Pinkie fished around in her wild poof of a mane, throwing out balloons, party poppers and the odd bunch of tangled confetti, all of which were pelted straight into Twilight's unamused face. Finally, her expression lit up, her eyes gleaming in pride as she pulled out the measuring tape. "Never leave home without it!"
"Right. Thank you," Thus the princess began to reiterate her explanation, this time making sure to clearly explain that the length stated on the measuring tape the first time was the circumference, and the second the diameter.
"Got it, Pinkie?"
Pinkie squinted at the pie, frowning in contemplation. "Interesting. Very interesting. But see, there's something I still don't understand."
"What could you possibly not understand?" Twilight almost screamed in frustration. Perhaps it was time to rethink her career in the education sector. "I haven't even got to the pi part yet! This is basic, primary school mathematics!"
Pinkie took in a deep, gasping breath. In retrospect, Twilight should have seen this coming. "The length of the circumference is 27cm, like you said, and the length of the diameter is 8.6cm, like you said, but what happens if the numbers are different?"
"Pinkie, I'm getting to that —"
"What happens if the pie isn't round? What if it's a square pie? Or a triangular pie?"
"Well, then it wouldn't work, but Pinkie —"
"What if it isn't a cherry pie? Does the flavour matter? If it was a blueberry pie, do we measure in smaller circles? What about a watermelon pie? Oooh, I've never had watermelon pie before, d'ya think it would taste good? Puff pastry or just normal pie dough? Hey, does that affect the circum-meter? Or the diam-ference? What about its lowest common denominator? Is the mitochondria still the powerhouse of the — ?"
"PINKIE!!!"
"Yes, Twilight?" Pinkie batted her eyelashes innocently.
"That..." Twilight wheezed, her face almost as red as the sticky sauce dripping from her hooves, which, in her sudden bout of anger, had plunged right into the cherry pie, "...has nothing to do... with the topic."
"Aw, but those are maths-y words!" Pinkie pouted. "Well, I'm not sure about that last bit, but it sounds smart, doesn't it?"
The purple alicorn threw her hooves up in the air in total defeat, sending flecks of cherry spattering into Pinkie's face, who eagerly licked it up with one swift motion of her tongue. "That's it! I give up! I'm never going to be able to explain pi!"
And with that, she flapped her wings and flew off into the distance, leaving Pinkie alone with the pie, the string and the measuring tape.
"Phew," Pinkie exhaled in relief. "That was close. Almost too close."
Reaching for the pie, she began to wonder what would happen if Twilight had found out her deepest, darkest secret. No — nopony could find out, nopony could know... lest Pinkie was banished to the outer reaches of Equestria for her thoughts, her words, her treachery.
She hated pi. Hated it with a passion, in fact. That silly, irrational number that everypony seemed to idolise had been taunting her for her entire life, stealing all the attributes she had always been proud of — her randomness, her comedic nature, even her surname! — all stolen by the vilest of all values.
I guess you could call that pi-racy!
No! Pinkie frowned and shook her head vigorously, clearing her mind. No. Now wasn't the time for jokes and puns. Pi didn't deserve that. Pi was too busy mocking her. Pi was the absolute scum of the earth!
How could everypony like it as much as they did? Who cares if it was an important part of mathematics? Mathematics, shmathematics. Applejack always told her all that fancy mathematics was useless, and the Element of Honesty wouldn't lie, now would she?
She took another furious munch at the cherry pie. She had to put a stop to this. This called for extreme measures!
But what could she do? She wasn't exactly the best mathematician, but she still knew there was no way to change it.
Change it... change it... The words echoed in her mind as she took the last bite of her pie, chewed and swallowed. That was it. She couldn't change maths, but there was definitely still something she could do...
"Knock knock!"
The castle doors creaked open as Twilight peeked out from behind. "Oh, it's you," Twilight smiled apologetically. "Pinkie, I'm sorry about how I acted yesterday. It was rude of me. Can you ever forgive —"
"Yes! Yes! Forgiven!" Pinkie chanted between bounces. "Listen, Twilight, I've heard of a much better value than pi!"
A wave of relief flitted across the princess's face. "So you figured out how it works. Good. Why, what did you find?"
"Tau! Just like pi, but double the fun!"
The relief disappeared. Twilight scowled.
"How. Dare. You."
Author's Note
Just a (very) quick little bit of randomness I wrote for Pi Day 2019. Hope you liked it! Let me know if you do, I might make a sequel for Tau Day... 
Have a nice day!
— WriterWings
