(The prompt: “[W]hen you have eliminated the impossible, whatever remains, however improbable, must be the truth …” (Sherlock Holmes, in Sir Arthur Conan Doyle’s The Sign of the Four))
(This was cuter in my head. Also the time limit got to me. I need to watch that.)
Princess Twilight Sparkle trotted down the stairs to the public portion of her library. It was early morning in Ponyville, with the sun shining and the birds chirping and everything just as perfectly idyllic as ever. She was freshly bathed and her mane was neatly brushed, and normally everything would be set for another wonderful, cheery day in her pleasant life here in the small town she'd come to love so dearly.
Except that there was a mystery hanging overhead, and unsolved mysteries tended to be a bit of a burden for Twilight Sparkle.
She trotted her way across the room and sighed a little before opening the door. Then she jumped backwards with a startled wail as she was met with a brightly shining pink face. "Hi Twilight! That's the best good morning wake up jump ever!"
Twilight took a second to calm her pounding heart before replying. "Good morning, Pinkie." She scrambled to her feet, and opened the mailbox on her door. Yep, right there in a plain white envelope, was yet another letter. Pinkie followed the letter with her eyes as Twilight magically hovered it over to read.
Pinkie Pie scooted in over beside Twilight, pressing her fuzzy mane against Twilight's as she read the letter out loud. "Dear Twilight Sparkle: By tonight you'll know who I am. See you at the party at Sugarcube Corner. Love, Your Secret Admirer."
Twilight flipped the letter over, and then placed it back in the envelope and set it down on the little table beside the door, next to the other six. "I've been getting these every day for a week! There's no information about who it is, and I don't know how they're managing to deliver these letters without anypony seeing them."
"Ooh! A mystery!" Pinkie plopped down a dark gray bowler cap on Twilight's head, and blew bubbles on a very nice looking pipe that she'd seemingly pulled out of nowhere. "You didn't find any clues, lowly assistant?"
Deciding to just play along for the sake of making things easier, Twilight shook her head. "There aren't any hairs or fur in the envelopes, the ink is just normal in every way, the paper is the kind they sell at the quill store, anypony could have gotten their hooves on it! Nopony sees them get delivered, and even putting a spell on the mailbox didn't reveal anything."
Pinkie adopted a look of careful consideration as she blew more bubbles that floated up into the air and popped around her head. She pulled the pipe away after a moment. "You're missing one clue, my fine filly-urp!" Pinkie burped, and a little soap bubble floated up and popped over her head. "Oh, don't inhale on toy pipes! Anyway, the party today's just us six! So it's got to be one of our friends!"
Twilight shook her head to clear it of the image of Pinkie burping soap bubbles, and sat down on a cushion next to her dining table. "Well... okay, that's true. It's just the six of us there, so that leaves five suspects."
"Oh! And Spike!"
Twilight made a face. "Ew, no. Spike wouldn't deliver letters like that, and just... ew!"
Pinkie Pie giggled, and then sat down next to Twilight, on the same cushion. "So let's figure out which of our friends is your secret admirer! Maybe it's Rarity?"
Straightening up a bit, Twilight entered her lecturing, facts and reason sort of mood. "It can't be Rarity. Even if she might try to send letters like this, these letters are all written by mouth, see? Rarity always writes with her horn, in calligraphy. Besides, she likes stallions, like Fancy Pants and Prince Blueblood."
Pinkie swished the pipe over to the other side of her mouth, blowing bubbles idly for a moment. "What about Fluttershy? She'd be too super-shy to just ask you!"
Twilight blushed slightly, and thought for a moment or two. "It could be her. Except that everything Fluttershy owns smells like animals, and these letters don't. Besides, she's always too busy feeding her friends in the morning to deliver these letters so far from her cottage."
"Ooh! She'd need to be super fast to get here and back in time between feeding the dawn animals and the mid-morning ones! Super fast like Dashie!" Pinkie threw out her forelegs to express the speed required, and fell over sideways onto Twilight, knocking them both to the floor. "What about Rainbow Dash? She's always over here reading and stuff right?"
Pushing her friend off of her, Twilight sat up, trying not to look too annoyed. "Well, yes, but she wakes up way too late to deliver these. Besides, Pinkie, Rainbow Dash would never write letters like this, she'd just fly right up and ask me!"
Pinkie nodded solemnly, and collected her fallen pipe from the floor, putting it back in her mouth. "You're probably right, lowly assistant!" Pinkie gasped dramatically. "That leaves Applejack! You could marry her and be a farmpony!"
Twilight looked down, blushing, at the letters. "Could it be Applejack? Maybe, but... Applejack's writing is terrible, and the letters look... more like..."
Bubbles idly wandered out of the end of Pinkie Pie's pipe as she sat next to Twilight. She was smiling even though she wore a look of concentration, by all appearances considering their friends to figure out which of them might be the culprit. But anypony who knew Pinkie, really knew her well, could see the little tension in her smile. That hint of worry on her brow, and the nervous moisture in her eyes.
Twilight bit her lip, realizing just how close she and Pinkie were sitting on this little cushion. "They... look like your party invitations. Pinkie, was it... you?"
Pinkie giggled nervously, and then hugged Twilight tightly. "Well duh! It can't be anypony else, can it?"
"But why go through all the trouble of writing letters if you're just going to show up and lead me to finding out it was you?" Twilight thought about pulling away from Pinkie's warm, tight hug, but just couldn't quite bring herself to do it.
"Because detective stories are no fun if they just TELL you who did it! And you should have known anyway, the pony who done it is always introduced just a few paragraphs in!"
Twilight blushed as Pinkie Pie nuzzled against her cheek. There was still a little tension in Pinkie's forelegs as they rested around Twilight's shoulders, and the nuzzling was just a little too close and a little too fast, like she was afraid she'd never get the chance again. Twilight had never believed in taking too long to make a decision. "Well... Pinkie Pie, would you... like to go to the party together, today?"