It was a chilly day outside. The kind of day you don't want to be out in. Which means, the perfect day for indoor activities! In Princess Twilight Sparkle's case, this meant learning and testing spells, followed with maybe some book reorganizing in what had become a royal library.
Twilight had set aside a room of the castle for this purpose, isolating the immediate effects of a cast spell from the rest of the world. This had the inconvenient side effect of requiring Spike to stay outside in case a letter needed to be received. (The last time Spike had been in this room for a protracted period, a later letter from Celestia mentioned a previous attempt having "bounced", which Pinkie Pie took as an invitation to hurl the scroll to the floor to see this phenomenon. Pinkie was sorely disappointed.) But a few scorches on the floor served as a stark reminder of its necessity.
"What is this writing?" said Twilight, turning a page in one of Star Swirl the Bearded's old spell books. "It looks modern enough, but...
"Don kciuq a evig dna, lleps siht esrever,
Ddo mees srehto ot dna, neve tonnac uoy fi."
Twilight waited a few seconds.
"Huh, nothing happened." She marked her place and closed the book, taking it with her. "Guess I'll get on reorganizing the library!" She let out a squee.
In the library, still holding the book, she unshelved all the other books, organized them, and reshelved them with the ease an average unicorn might have in lifting a hayburger to her mouth and holding it as she eats.
Well, almost.
For one thing, she found difficulty in that no two rows on the same bookshelf unit would allow itself to be made the same length as another. And there was another limitation that she found herself unable to figure out just yet: for no natural reason, she wouldn't be able to place a final book on a shelf.
And it was in the process of struggling against this limitation with physical force that Twilight found herself when Rainbow Dash entered through a window. To her credit, she opened it first, so it didn't result in a hazard for bare hooves and rear claws.
"Hey, Twilight!" she said. "You got the latest Daring Do yet?"
"Oh, sure," said Twilight, grinning. "Coincidentally, it's on the very shelf I'm struggling with." She went back to trying to force the book back, putting all her weight against it. But it was to no avail; the book would not slide into the shelf.
"Having trouble reshelving a book?" asked Rainbow. "That's new. At least, for a book you don't want to read at the moment. Mind if I try?"
"Be my guest," Twilight said, standing aside to let the mare at it. Rainbow picked up the book and easily put it into place.
Twilight stared a few seconds, agape, only breaking her gaze with a blink. "...How'd you do that?" she asked.
"There wasn't any trick to it, Twilight," said Rainbow. "I just... picked it up and put it on the shelf like normal. ...Are you okay...?"
Twilight sighed. "I think so," she said. "I just... couldn't do it. Why couldn't I do it? It doesn't make any sense!"
"That's easy!" said Pinkie, sticking her head through the space between a book and the end of its shelf. It happened to be the smallest one of the spaces. She emerged fully and hopped safely to the floor. "You just felt like being uneven today!"
"If I felt like it," said Twilight, "I wouldn't have been trying so hard to put this book here."
"Wait, what do you mean by uneven?" asked Rainbow.
"Well, just look at it," said Twilight. "No two rows on the same bookshelf at the same length! I mean, they're always going to be at least a tiny fraction off, but--"
Pinkie giggled. "That's only part of it!" she said. "Look again; every shelf has an odd number of books! Oh, except for the one Rainbow added a book to!"
Twilight looked over the shelves, doing a quick book count.
"Wow," said Twilight. "You're right! I never noticed that!"
"Uh-huh!" said Pinkie. "But never mind that! It's the perfect day to play in the snow!"
"Snow?" said Rainbow.
"Snow!" said Pinkie. "Yuki! Sheleg! Qaniigix! Eira! Iqhwa! Neve! Barf!" The two non-earth-ponies stood there, processing the flurry of foreign words.
"Oh, that's right," said Rainbow, "today was a scheduled snow day, wasn't it... Wait, 'barf'? Ewww..."
"Snow... neve... even..." said Twilight. "I've got it!" She grabbed the spell book in her aura... and found that she could not remove it from the shelf.
"Leave this to me, Twilight," said Rainbow. Twilight released the book, and Rainbow flew up and grabbed the book, removing it and bringing it down to Twilight.
"Thank you, Rainbow Dash," said Twilight, taking it in her aura and opening it to the marked page. After a good look at the incantation, she said, "Aha!
"If you cannot even, and to others seem odd,
Reverse this spell, and give a quick nod."
Per the final instruction, she gave a quick nod. Again, there was no immediate indication that anything had happened, but Twilight immediately set to reorganizing her books. She found she was again able to make rows of the same length -- within tolerances, of course -- and it once again didn't matter if the number of books on a shelf was even or not.
"Well!" said Twilight "With that done, I believe I will join you, Pinkie... Pinkie?"
"Yes?" Pinkie said, poking her head out from in between two books. She was wearing a woolen hat and scarf, and when she poked a foreleg out to present Rainbow with her gear, it became apparent that she was wearing hoofwarmers, as well. Twilight had to laugh at the sight, and Rainbow and Pinkie joined in, a laughter that would continue as they enjoyed the snow to the fullest.