//------------------------------//
// An Explanation of Unicorn Magic 
// Story: The Ties that Bind 
// by the7Saviors 
//------------------------------//

Later that same day after lunch, the three fillies were called back into Ancient Scroll's study for the mana evaluation he had mentioned earlier.

Twilight and Sunset were of course curious to see just what this thing was that he had picked up, but Ancient Scroll noticed that Trixie still seemed out of sorts from his announcement. He frowned slightly as he laid eyes on the blue filly, but continued on regardless - making a mental note to talk to Trixie one-on-one later.

"Alright, girls," he began, sweeping his gaze over each of them, "now that you've got something in your bellies, it's time for that evaluation I promised, but first," he eyed the three contemplatively before continuing, "I'm sure you all know the answer to this question by now, but humor me for a bit."

He leaned forward and clasped his hooves together over the desk as he continued.

"Who here can tell me how magic works when it comes to unicorns and how mana is measured?"

Twilight's hoof shot up first, quickly followed by Sunset's.

Trixie didn't even bother as she looked off to the side distractedly.

Ancient Scroll gave the blue filly a brief side glance before turning his attention to Sunset.

"Alright, Miss Shimmer, let's hear it," he said with a nod of encouragement.

Twilight huffed and lowered her hoof as Sunset began her explanation.

"Unicorns use magic to actively influence the world around us, unlike earth ponies and pegasi who use their magic passively and instinctively," she explained, "while the mana of earth ponies and pegasi is suffused through magical 'veins' spread throughout their bodies, unicorns pull mana from a magic 'wellspring' within our chest. We pull mana from this wellspring using our horns as a natural conduit to cast spells."

"Very good," Ancient Scroll responded with a proud smile. He turned to the eager lavender filly, "and the measurement of mana, Twilight?"

Twilight grinned, standing up a bit straighter and raising a hoof.

"Mana for unicorns, pegasi, and earth ponies is measured in thaums, but a unicorn's wellspring has a unique measurement for depth -  lyms. The higher the thaum count, the more mana a pony has. The higher the lym count, the deeper a unicorn's wellspring. A lot of mana means you can put more power into spells, and a deeper wellspring means a higher mana overflow threshold."

Ancient Scroll nodded in approval before looking back to Trixie, who had been staring at nothing in particular.

"Trixie."

The blue filly snapped out of whatever thoughts she had been ruminating on and turned to Ancient Scroll, blinking owlishly. Ancient Scroll's frown deepened and the other two fillies eyed Trixie worriedly.

"Trixie, why don't you go lay down for a bit and get some rest," Ancient Scroll suggested, "we can pick this back up when you're-"

"No," Trixie cut in, shaking her head to clear her thoughts, "Trixie is... fine. Just some... stray thoughts is all," she tried - and failed - to give them all a reassuring and confident smile, "you can continue with the lesson, Uncle Scroll, Trixie is all ears."

"Okay then..." Ancient Scroll replied, thoroughly unconvinced, "tell me, Trixie, what is mana overflow and how does it affect unicorns?"

Trixie said nothing for a moment as she closed her eyes and hummed to herself in thought. After a minute she open her eyes, cleared her throat and began speaking.

"Mana overflow happens when the mana used to cast a spell surpasses a unicorn's mana overflow threshold," she paused to collect her thoughts before continuing, "a unicorn having more mana than their wellspring can hold isn't a problem in and of itself, as the exa... excess mana is released passively through small pores in the body and dissolved into the ata... atma... atmosphere."

Sunset looked impressed as Trixie continued and Twilight just smiled, remembering how Trixie had told her about her good memory. Ancient Scroll himself was happy to let Trixie continue her explanation as long as it kept her mind from wandering to whatever she had been thinking about before.

"The real problem occurs when a unicorn starts trying to cast a spell with more mana than their wellspring can hold," as she got more into it, Trixie began to make dramatic gestures to go along with her explanations, "when that happens, the active mana traveling from the wellspring begins to overload the horn and that could have det.. de.. tri... detrimental effects on the brain."

Trixie tapped her own horn as she began to strut about in front of the other two fillies.

"This could range from simple magical burnout to sudden brain death depending on how much mana was pumped into the spell, however," the blue filly suddenly stopped and jabbed at the other two fillies, "this can be easily avoided by dialing back the mana you use to when casting that particular spell."

"Excellent work, Trixie," Ancient Scroll said, practically beaming with pride, "you know, you don't give yourself enough credit, little filly."

Trixie huffed and stuck her muzzle in the air haughtily - though the others didn't miss the red tinging her cheeks.

"T-Trixie has no idea what you're talking about!" she scoffed, "she gives herself all the credit she deserves!"

Ancient Scroll chuckled softly. He knew the truth of the matter, how the blue filly was constantly pushing herself to try and keep up with the other two fillies. In reality, Trixie was just as brilliant as the other two, but for all her bravado and self-aggrandizing, she lacked the self-confidence to see herself for the smart filly she really was. 

"Alright I think that's enough of the basics for now," he said, lighting up his horn and opening the door across the room from his desk, "before I get to the magic evaluation, I need to speak with Twilight alone for a moment, so Sunset, Trixie? If you girls would?"

Sunset and Trixie looked at Twilight with curiosity and confusion and Twilight could only reciprocate the feeling with a shrug. The blue and orange filly glanced at each other, gave a shrug of their own, and trotted out of the room before Scroll gently shut the door. 

Twilight turned back to Ancient Scroll to find his gentle smile had been replaced with a slight frown as he gazed down at the lavender filly. She swallowed nervously and squirmed under his stare, afraid she had done something wrong.

"Y-You wanted to tell me something?"

Ancient Scroll continued to eye Twilight for another moment before he sighed and spoke in a solemn tone.

"Remember when I told you I'd let you know when I discovered something about your... situation regarding the miraculous surge?"

Twilight blinked in confusion before her eyes widened in sudden realization.

"Did you find anything out?" she asked, stepping closer to the desk, "is there something wrong with me?"

"Well, not exactly, little filly," Ancient Scroll replied slowly, "nothing's wrong, but your situation is definitely a... unique one."

"And?" Twilight pushed, "what did you find out?"

"I can't say anything for sure without running a few tests," he said with a shake of his head, "that's actually part of the reason I decided to do this magic evaluation, but from what I can guess, you have a lot of mana, little filly," he leaned forward over his desk and eyed the filly critically, "a lot of mana - almost to a dangerous degree."

Twilight began to shift uncomfortably as she thought about the implications.

"I couldn't find anything on the odd phenomena regarding your magical affinity - in fact all I really have right now are educated and uneducated guesses, but I believe the only way you could've survived that surge is if you somehow forced your wellspring further open at the last second to accommodate for the increase in mana output."

Twilight fell to her haunches and furrowed her brow in thought.

"But how is that possible?" she muttered, "unicorns shouldn't have control over their wellsprings at all. They grow and develop on their own, naturally over time."

"I'm not too sure, filly," Ancient Scroll responded, "there is a theory floating around in the thaumatologist community that magic miracles are possible because the still developing wellspring has a sudden 'spasm', causing it to expand temporarily, but the theory doesn't explain the sudden increase in mana."

He shook his head and looked at Twilight, stroking his beard contemplatively.

"I think it's different with you, though," he continued, "I think with you, the mana needed is already there, and your wellspring..." he went quite for a moment before sighing in defeat, "I don't know, little filly. Like I said all I have are guesses at the moment."

Twilight sat there, mulling over everything Ancient Scroll had said.

"Do you..." she hesitated and Ancient Scroll raised an eyebrow. She looked down at her hooves before looking back up and speaking once more, "do you think we'd have anymore luck if we were to go to Canterlot?"

Ancient Scroll hummed to himself and gave a slow nod.

"I suppose we would," he said carefully, "they do have some facilities open to the public over at the Institute and we could browse some of the books they have there," he eyed Twilight warily, "what brought about the idea?"

Twilight rubbed the back of her neck sheepishly.

"Oh, nothing in particular," she said with a nervous chuckle, "I just thought Canterlot would be a better place for research what with the Institute of Magical Research and Development being located there."

"I see..." Ancient Scroll answered with a raised eyebrow. He wasn't fooled however.

He knew exactly why Twilight had brought up going to Canterlot.

Nevertheless he decided not to call the filly out on her lie and instead motioned to the door.

"Well, we'll discuss this more later, little filly," he said as he leaned down beneath his desk to open one of the drawers, "in the meantime, can you get the other two for me?"

He continued to root around in the drawer as he spoke.

"I believe it's about time we got this magic evaluation underway."