Next morning in the Vault, Celestia tested Twilight on the spells the little filly already knew. They began with levitation, which Twilight managed reasonably well, but only for certain objects: books, quills, sheets of paper, and objects the size and shape of her toys.
"To truly grasp levitation," Celestia told her, "one should be able to levitate anything on sight. Like this..."
Twilight whooped and then giggled as she was suddenly bounced into the air and turned in a slow somersault. "Wheeee! Again, again!"
"In a bit, if you focus on the lesson."
Landing on the ground, Twilight nodded eagerly.
"Now, let's start with something you probably haven't seen." A flicker of magic coalesced into a glistening translucent sphere.
"Woah!" Twilight stepped closer to look at it. It was something between glass and gossamer, like a soap-bubble but with no colors to it. "What is it? A crystal ball?"
"Close -- it is a perfect form. Every object in Equestria is based on perfect forms like this one. Most are combinations of them, so you'll need to become familiar with all of them. Take a good long look at this one, and let me know when you're ready."
Twilight thunked down on her hindquarters in front of it and stared and stared, looking for details, the way she had with the doors. But in this case there really was nothing more than the sphere's size and shape. After a minute or two she got up, and nodded.
"Now, let's make sure you can't cheat." A blindfold appeared, and wrapped itself around Twilight's head.
"Hey! I can't lift it if I can't see it!"
"Twilight, you should be able to lift an object wherever it is, so long as you know two things: exactly what it looks like, and exactly where it is in relation to yourself. You have both in this case. And we'll keep it simple. I only want you to lift it straight up and down. Oh, and let's make it a little more interesting..."
Twilight squealed as her hooves left the ground again. "Now I don't know where the floor is!" She protested.
"You shouldn't need to. The sphere is exactly where it was, floating right in front of you. See if you can find it."
Twilight tensed, her face scrunched up and her horn flickering weakly.
"Twilight, what are you doing?"
"I'm focusing! Like I always do for my spells!"
"A spell isn't something that you do, like lifting a load with your shoulders. It's something you make happen, by creating it and then powering it with your magic. Once you're better at it you should be able to lift objects without the least physical effort. Try to relax, and just think about the target of your spell and what should happen to it. Try to see it in your mind before you power it with your horn."
Twilight let out a breath, then steadied herself. Celestia watched as she lowered her head, thinking. And then her magic sought for the sphere. First it was too low, then too high, then to the left, then the right. She connected, and the sphere gave a soft ping.
"Yay! I found it!"
"Very good. Now remember: lift it straight up and straight down."
Twilight nodded. At first she tensed, but then she remembered, and deliberately relaxed herself. And the sphere juddered and began to move. Twilight lifted it a decahoof, then lowered it again.
"Excellent," Celestia complimented her. "Now do it again, and keep doing it until I tell you to stop."
As Twilight complied, Celestia tweaked her own magic. Both Twilight and the sphere began to gently rotate in space, rolling over and over.
"Woah! What's going on?"
"Nothing that should concern you, Twilight. The sphere is exactly where it was, relative to you. Center on yourself, stay focused on it, and keep lifting and lowering it."
"'Kay..." Twilight said, doubtfully. Yet she persisted, even as Celestia added another axis of spin, and then another. Twilight kept the sphere moving up and down steadily, as both she and it went round and round in all directions. Once, it almost got away from her, but Celestia gave it a gentle nudge to steady it until Twilight got a grip on it again.
"Now," she said, "instead of up and down, move it straight to the left, and straight to the right. That's correct! Keep at it. You're doing splendidly."
When Twilight had accomplished that, Celestia had her try sending the sphere out and back. It was a little harder and Celestia needed to steady the sphere more often, but she could feel Twilight rapidly getting the hang of it.
Finally, she slowed Twilight's rotation to a stop and settled her hooves on the floor again, removing the blindfold as she did so. Twilight wobbled unsteadily for a few moments as she got her land-legs back. "Oooooh! Cadance said I shouldn't have a big breakfast this morning. Now I know why!"
"Do you feel all right?"
"Yeah, I'm fine! That was fun!"
"I'm glad. Because in the end you're going to be royally sick of it. But it's important, trust me. To put together spells you need to take them apart into their elements, study the elements until you know them expertly, and then put them together, without being distracted by aspects that don't matter. And that means a lot of practice. So it's going to be light meals in the morning for while, I'm afraid. Now, let's look at some of the other spells you know."
They tried Twilight on luminance spells, which she could do relatively well. Celestia pointed out that the light should not be limited to Twilight's horn, but should be manifestable anywhere around her. And that Twilight should be able to choose its color at will. "Again, it's a matter of breaking the spell down into geometric position and hue values, and combining the elements you want."
Then they moved onto deflection spells, and here Celestia was surprised to find Twilight's abilities relatively weak. She could put up small barriers to slow an object flying towards her, but many unicorns could do that simply by reflex. She wasn't able to stop or deflect physical objects at all, and she had little skill with shielding against magic itself.
"I guess it's because I've always counted on Shiny's shield spell," Twilight admitted. "I never had to learn to do it myself."
"Then we should start you on shield spells as part of your practice work. You'll find they're not too hard, once you understand the basics of deflection and the perfect forms. Then you just put them together the way you need them."
"Uh, Princess Celestia?"
"Twilight, if you have a question, you ask it, remember?"
"Sorry! I was wondering -- is that how my brother's shield spell works? Like, combining the perfect form of a sphere with deflection?"
"Very good, Twilight! That's exactly how it works. But don't sell your brother short. I understand he can use the shield to block both physical force and magical energy, and he has excellent control over what is blocked and what is allowed to pass through. He may only know the one spell, but his mastery of it is quite impressive!"
"Wow! Um... is it okay if I tell him you said so?"
"Of course! When I think my subjects are doing their jobs well, I have no objection to saying so."
"Oh good! I wanna see him blush!"
"Ah hah."
Celestia skipped over offensive spells, transformation, and particularly invisibility, not wanting to add to Cadance's burden in managing Twilight. There would be time for those later.
Then she turned to healing and emotion spells and discovered another gap in Twilight's abilities.
"Well..." Twilight rolled her eyes, embarrassed. "I never really needed healy-feely spells. I don't have many friends, aside from my big brother. And when you use magic to make someone feel better, it never lasts, right?"
Sadly not, my little pony, Celestia thought to herself.
"Twilight, I begin to see why you had so much difficulty with magic during the exam. Spells are a combination of structure and magical energy. You use what you know to construct the spell, give it shape and purpose, but you also need emotional experience to apply magic to power it. Otherwise, you would not be able to judge whether the spell will help or hurt. And when applying emotion spells in particular, you need to know a lot about who you're helping with them to do them properly. You have a very analytical mind, Twilight, and can construct spells beyond your years. Yet you need a control of magic that is equal to that capacity, in order to power them properly."
"So... because I'm not good at feely spells, my magic gets away from me?"
"In a sense. We should find ways for you to practice those kinds of spells too, so you'll have the control you need."
"Ooookay," Twilight said. It was clear this prospect was less appealing. Celestia made a mental note to ask if Cadance would mind taking a morning session now and then. It made sense for Twilight to learn from the best.
They passed on to other spell types and Twilight showed similar unevenness, excelling in some areas, and having passed others over completely in her self-study.
Celestia left firefly spells for last, in case they ran out of time, and because they made a pleasant diversion. She demonstrated a few, like materializing little animated ponies out of magic that danced and sang songs. And then she told Twilight to look up, just as a large glob of water smacked onto her nose. Shaking it off, Twilight asked to see how that one worked, and was sad to find she lacked the materialization skills needed. Celestia had to promise to recommend a book Twilight could read to help her learn the basics.
Neither of them had realized how quickly the time was passing until they heard a quiet tapping from outside.
"Let me do it!" Twilight said. She turned around away from the tapping sound, and summoned the Vault doors. They swept open, revealing Cadance standing in the corridor beyond.
Celestia nodded at that. She's definitely improving, and willing to try things that might fail -- a very good sign.
"My apologies, Princess," Cadance said. "It's time for Twilight's lunch. Though if you need more time I can come back."
"No, I think we've made very good progress for today. Twilight, I'll send you a note with some references I'd like you to look up and read, and we'll pick this up again tomorrow. Okay?"
"Okay!"
Celestia reluctantly stood outside the Vault, watching Twilight bounding off after Cadance, chattering like a bird, until they both turned the corner. And then she turned to the rest of her own agenda for the day.
"... and she taught me about firefly spells, too!" Twilight was telling Cadance.
"Really?"
"Yeah! And she mentioned I should ask you about the really good one you know how to do!"
"Oh, yeah! That is a good one. But I can't show it to you right now."
"But why not?" Twilight demanded, with a sad look.
"I tell you what, Twi," Cadance said, mysteriously, "if you can wait until bedtime tonight, you'll see why. Deal?"
Twilight brightened up immediately. "Okay!"
That evening, just after the Lowering Ceremony, Cadance dressed Twilight and herself in warm cloaks against the chill and then led her through the doorway into her room, and out onto her balcony, which looked down on the Canterlot Royal Gardens with its hedge maze, stone statues, and zoological park.
"Now hold still, Twi," Cadance warned. And Twilight felt herself lifted off the stone flooring, held firmly in Cadance's magic, and settled onto Cadance's back.
"No fidgeting back there, now! I need to focus!" Cadance spread her wings and lifted off the balcony, swooping down into the darkened gardens below. She thumped down on the grass, stumbled a bit then recovered, and finally swept Twilight off her back and set her on her feet.
"Now," she whispered, glancing around. "we're really not supposed to be wandering around down here this late at night. But if you promise not to tell the guards, I won't either!"
"Okay!" Twilight whispered back, delighted to be in on mischief.
They trotted through a gate into an open expanse of grass, surrounded by shrubs and trees. Cadance came to a halt in the middle of the open area, then glanced around to check that they were out of sight of the main path.
"All right," she said, "this isn't technically a firefly spell. It's really an emotion spell and it takes a bit of practice to pull it off. Do you know why fireflies flash, Twi?"
"Sure, to signal to other fireflies. It's how they find each other."
"Right! I forgot who I was talking to. Well, when a firefly flashes, it's basically signalling 'Here I am! Come be with me!'. So all I have to do is give them a little nudge with my love spell, and..."
Her horn sang with power, and its magic spread out across the open space.
And every firefly lit up, in the grass, on the branches of the bushes and trees, and in the air, twinkling and flashing, a snowstorm of gently glowing green embers.
Twilight spun slowly around, looking at them all. "Wooaaahhhoow!"
Gradually the fireflies winked out again, though a few of the more persistent continued flashing.
"I can't really do it more than once," Cadance said. "It's not fair to the fireflies. But what do you think, Twi? Is that a good 'firefly' spell, or what?"
"It's the best!" Twilight agreed.