Because of the way Karyn’s new apartment was situated, all in a single line with the bedroom at the back, light only entered from one side. The window in the bedroom itself was small, frosted, and, in truth, needed a good scrubbing. It also had thick curtains that Gayle had put up when she lived there. The result of all this was that it made for a very dark room.

Karyn hadn’t taken much notice during the week. When getting to an early class depended on getting up, she used her alarm clock and thought nothing of it. But on Sunday, the weather turned cloudy for the first time, and there was even less light. Karyn wound up sleeping all the way through Derpy’s arrival.

To her credit, Derpy went invisible as soon as she saw what seemed an unoccupied dwelling, and searched carefully. Once she saw Karyn’s head on a pillow, she softly shut the door and returned to the kitchen. She put on coffee, finding the maker a bit tricky to use with hooves, and waited.

Twenty minutes later, Karyn finally emerged, her hair mussed and tangled, rubbing the sleep out of her eyes. “Derpy, you’re here.”

“Yes. Rough week?”

“Not really. I even went to bed early. I think I woke up at four in the morning and figured that was too early, and my body thought that an entire day had passed. Ooh, coffee.”

She picked up the cup that Derpy offered her and, after checking its temperature, took a sip. The heat alone opened her eyes wider.

“I figured since it was your first week back at school, they worked you hard and made you tired.”

“No, not really. The first week of classes is never that interesting, to be honest. They give out the syllabi and go over it, and only if the class is extra long do they actually start a lesson. Plus a lot of the students ditch the first class, and the professors know it.

Derpy stared over her cup. “Did you ditch?”

“Certainly not! Attendance is key for these classes, especially this year. If I am going to ditch, I’m not going to waste it on the first week.”

That did not entirely please Derpy, but she let it pass. “And you got right into your books, I’m sure.”

“Well, not exactly. The classes were really boring, but I couldn’t leave or waste time browsing on my phone—another thing that professors hate—so I had a lot of time to think.”

“Oh? About anything interesting?”

Karyn took a long pause. To Derpy’s view, it wasn’t a rehearsed speech, but it was clearly something that she had intended to bring up. Some of her turns of phrase were probably chosen beforehand and not made up on the spot.

“Mostly about my reaction ever since Queen Chrysalis told me that I was now a changeling. I’ve been so scared of coming to terms with it, because it makes me different from everyone else. Among humans, that’s always a dangerous place to be. If anyone in that class knew I was a changeling—well, you saw how Gayle reacted last week, and that’s probably the best-case scenario.

“But it’s who I am now, like it or not, and I’ve got to face that fact. What that means is that I want to actually try using my powers more, not just for covering for you with Gayle or avoiding having to put on makeup in the morning. Which I’ve given up doing.”

“You’re never wearing makeup again?”

“No, I mean I gave up using my powers for that. But maybe I should keep doing it. Like I said, I’m different, and isn’t it great to be different?”

“Don’t ask me,” Derpy said, but with a humorous tone in her voice.

“Beyond that, though, I think there are ways that I could use them for more than just disguising.”

“Didn’t we already try that with Scootaloo? You don’t gain the powers of whatever you change into.”

Karyn stood up from the table. “We did, but that can’t be true all the time. Let me illustrate.”

She handed her cup to Derpy. “Take this and put it on the top shelf of the cabinet.”

Derpy acceded, remarking, “I think you mean demonstrate, not illustrate.”

“You’re right. I must still not be fully awake. Never mind. Now, we know that I can’t fly up there and get it, even if I’m a pegasus pony.” She switched to the pony version of herself and flapped her wings uselessly. “But what if instead I just changed into a taller human?”

Karyn concentrated again. This transformation was less familiar to her, but after a few seconds the circle of green light came and went, leaving a tall black woman in its wake.

“You changed a lot more than that,” said Derpy.

“This is an actual copy of a basketball player I saw on TV during the week. And now I’m tall enough to get the cup back.” She did so, then switched back to herself. “I was able to do something in another form that I couldn’t as myself. That’s what I mean by using my powers more.”

“I see. You should go and look at a lot of people so you have more images. Like if you found someone really strong.”

“Maybe. I don’t know if that would work. Height is inherent to people. Am I really copying their muscles? But beyond that, I thought of something I wanted to try, but I wanted you to be here in case things go wrong.”

Derpy spread her wings and pointed at herself. “I’m right here.”

“Yes, but I thought of it during the week. This involves a couple of new things. A partial transformation, and dealing with inorganics.”

“What do you mean?”

Karyn went into the bedroom and came back with a small duffel bag. “Well, for example, I can form clothes even when I’m not wearing any. I’ve never paid attention to what they’re made out of. They could be wool, which would come from an animal, so that’s still just a regular transformation. Can I make something out of cotton, which is a plant? What about polyester?”

“I’ve never heard of that plant.”

“That’s not even a plant. It’s a synthetic fiber made of plastic. But here’s what I want to do. I used to take this bag with me to the gym, and I kept this padlock to keep it in a locker. It uses this little key, right? What I want to do is see if I can’t mimic the key and use it on the lock.”

Derpy opened her eyes wide. “Ooh, that would be useful.”

“But I want you here in case something goes wrong and I turn my whole body into the key or something, and you have to race me back to Equestria and beg Queen Chrysalis to help turn me back.”

“I don’t want to beg her for anything. She’s kind of a jerk.”

Karyn shrugged, and put the lock and key down. “The key’s made of metal—steel or aluminum or nickel, probably, but I’ll try doing it with iron. That’s something I have in me already.”

She held out her finger and stared at the key. Derpy had no basis to understand how Karyn was thinking, but she concentrated herself and hoped that it worked out. Finally, after thirty seconds or so, the green fire appeared again. It surrounded Karyn’s entire body, and Derpy was afraid that Karyn’s prediction was coming true. But when it dissipated, there she was with a dark iron protrusion from her finger. In substance, it reminded Derpy of the wrought iron gates and fences that sometimes existed in Equestria. But it had the contours of the key.

“I think it worked,” she said.

“It certainly changed. Let’s see if it does the same job.”

She closed the lock, slipped her modified finger in, and crossed the fingers on her other hand. Looking at Derpy, she turned it. A click was heard, and the thick metal ring popped open.

“You did it!”

Derpy’s excitement quickly changed. Karyn turned her finger back and pulled it out. She was visibly wincing.

“Yow! Ow, that hurts.” She focused and transformed back to her full self. When she held up the finger, it was red and swollen.

“What happened?”

“I don’t know. But it feels like…well, honestly, it feels like I shoved my finger into a lock and turned it.”

Derpy grabbed the hand in her hoof. “Can you bend it at all?”

“Yes. I don’t think anything’s broken, but it feels like a strain or a bruise. I don’t think I’ll be trying that again.”

“I guess not. Still, it’s a useful ability if you really need it. If you found yourself locked out, you could take the pain instead of calling a locksmith.”

Karyn nodded as she poured cold water on her finger. “The problem with that is that I copied the key, not the lock. I don’t know if I’d have the ability to change myself into something I didn’t see. That’s why I want to do more tests, that hopefully aren’t this painful.”

“I wonder, can you change it so that it doesn’t hurt?”

“I don’t think so.” She concentrated, and once more the green flash was seen. “Yeah, see, it looks fine now, but it still hurts.” She changed back.

The sound of footsteps was heard outside. Derpy backed off from the window, which was high enough off the ground that no one could see in. Karyn walked over and peered down at the sharp angle. Gayle was out in the yard. Immediately, she looked up, waved, and winked.

“Is she there?” asked Derpy.

“Yeah. There’s no reason for her to walk by my window to go out and pick up the paper. She’s spying.”

“I guess we should be a little more discreet.”

Karyn nodded. “We could take it to Equestria. Nopony is weirded out by magic there.”

“They might be by changeling magic. But we’ll go if you like.”

“Let me just clean up first.”

Derpy found out a moment later that she meant cleaning herself, so it was left to Derpy to wash the coffee mugs and pot, then sit and wait for Karyn to put on her face.

“Maybe you should be using your magic for that. Come on, get on my back.”

Karyn mounted, and Derpy pulled out her spell. As they glided down, Karyn said, “Hmm.”

“What?”

“Well, I can’t fly as a pegasus, but I wonder if, if I can find the right form, if I can glide.”

“Don’t try it. If you’re wrong, or if there are side effects like the key, it could be a problem.”

They landed and entered Derpy’s house, which was pristine and orderly as always. The cat was waiting for Derpy, and seemed happy to have her back sooner than usual.

“Now that’s a question,” said Karyn. “I wonder if I could do a cat transformation.”

“I don’t see why not. It’s alive, and it’s not a partial transformation.”

“But it is a lot smaller. What I want to find out is if I still have to follow conservation of mass.”

Derpy looked at her with a confused expression.

“When I became the taller girl, I could reach higher, but I didn’t weigh myself. If I was the same weight, then it would put a limit on things I could do. It would mean that, if I became an elephant, all my molecules would be farther apart. I might turn into vapor, drift away, and kill myself.”

“So don’t do that. Turn into something smaller like Muffinhead.”

“But that could be a problem too. If I get too much mass in a small place, will I be able to move? Will I explode?”

Derpy held up a hoof. “Hang on, though. Elephants weigh more than humans. Why wouldn’t you just weigh the same?”

“Well, where would the extra mass come from? Matter-energy can’t just be created out of nothing. That’s a law of physics.”

Derpy was not usually given to sarcasm, but she stared and said, “Right, when using the ethereal force of changeling magic to alter the very fabric of reality, it’s important to keep the laws of physics in mind.”

“Oh, I guess that counts as an open system, huh?”

“Well, go on and try a size transformation to find out.”

“We need to know my current weight first. Can you get your bathroom scale?” Karyn knew that Derpy had one.

“Here you go.”

She stepped on. “OK, I weight one-oh-eight right now.”

“I am so jealous of you.”

“And that gives me a good benchmark of something that I can turn into and not be too far off my current weight.” Karyn focused, and repeated the action that was her first ever transformation. A second Derpy stood in the room. She stepped back on the scale.

“What does it say?”

“Well, the good news is that indeed changeling magic can alter mass.” She stepped off and turned back into her old self. “The bad news is that you have to go back on your diet.”

Derpy glowered at her, but any further argument was cut off by a knock at the door. She went over and opened it. “Hey, neighbor.”

“Is everything all right in here? I saw—oh, you have company.”

Derpy stepped aside to reveal to Karyn a pretty-looking unicorn with a two-tone mane and tail. As she entered Karyn could also see her cutie mark, an hourglass.

“Nice to meet you.” She racked her brain to think of her name. “It’s Minuette, isn’t it?”

“No, my name is Colgate. Pleased to meet you.”

Derpy introduced Karyn by name, and Colgate nodded. “I was passing by and I saw a pair of green magic flashes. Since I knew that a pegasus lives here, and, if you’ll forgive me, Derpy, you don’t usually have guests and go away on Sundays anyway, that some unicorn might have broken in.”

“Nopony would do that, I’m sure,” said Derpy. She went on to explain, in detail, about Karyn, humans, their original adventure dealing with changelings, and finally the illness that led them to discover Karyn’s powers. “And now, after some trepid…trepidat…after she didn’t want to in the first place, Karyn wants to become the best changeling she can be.”

Colgate stared, wide-eyed. “Wow, Derpy, that’s an incredible story.”

“Yeah.”

It was Karyn who noticed how nervous she seemed.

“Do you mind very much if we experiment?”

“No, not at all. Why should I mind if you’re trying experiments with dark magic in Ponyville?”

“Hey! Karyn’s not dark!” Derpy stomped her hoof in anger.

“I didn’t mean that. I’m sure that humans can be good or evil just like anypony. But changeling magic is definitely dark magic.”

“It is not!”

“Hang on,” said Karyn. “Can you explain the difference between dark magic and…light magic, I guess?”

“Dark magic is used for evil purposes. Everypony knows that.”

“Ah, so technically there’s nothing inherently good or bad about magic. If somepony’s trapped under a cart and you levitate it off of them, that’s good magic. But if you levitated the cart over that same pony and dropped it, that would be dark magic.”

Colgate squinted. “But changeling magic is basically lying to try to steal somepony’s love. How could that ever be used for a good purpose?”

“In the first place, I don’t have to steal love to survive. Thankfully. In the second, I wouldn’t use my powers to hurt anyone. Yeah, I could disguise myself as, like, a bank manager and go in and take all the money, but I wouldn’t do that because it doesn’t belong to me.”

Colgate trotted around Karyn, prodding her with her horn before finally using some of her own magic. It did not seem to produce any visible results.

“What are you doing?” asked Derpy.

“This is a very old unicorn spell. It’s taught to us when we first learn our magic. It tests the purity of another’s heart.”

“I’ve never heard of that spell.”

“Unicorns have very little occasion to use it.”

Derpy glowered. “But you feel that it’s worth it to use it on my friend?”

“If your friend weren’t using changeling magic—“

“Hang on, I know this, it’s, um…circular logic! You only think the magic is bad because you’re doing the test, and you’re only doing the test because you think it’s bad.”

“No, Derpy,” said Karyn. “I understand. There’s a reason to be nervous, when changelings steal love and impersonate princesses and such.”

Colgate snorted. “In any case, you totally passed the test, so it’s a moot point.”

Derpy smiled, but then mumbled, “I never get to talk about stuff when somepony talks about moot points.”

Karyn tried to mollify Colgate further. “One thing that we’ve discovered is that there are a lot of ways a transformation can go wrong. I’m actually impressed now that Queen Chrysalis was able to fool as many ponies as she did for as long as she did.”

“How do you mean?”

“Well, take for example my teeth, which I know that you’re interested in. I could make them as white as you like, but unless I actually brush them, they’ll still get cavities and decay.”

“Well, of course you would! Everypony has to brush.” Colgate gritted her teeth.

“But if there were some magic that could clean teeth automatically—“

“There can’t be. It’s impossible. Everypony. Has. To. Brush.”

It seemed that Karyn had found the one thing that Colgate cared about more than evil magic taking over Equestria.

“I always brush, though. My mother drilled it into me when I was little. Heh, get it? Drilled it into me?”

“That’s good, though.”

“I always made Dinky brush, too,” said Derpy.

“Anyway, show me more of your changeling powers.” As members of the brush-their-teeth-frequently club, Karyn and Derpy had won Colgate’s trust.

They talked through all the research they had done, and upon Colgate’s inquiry, told her also about the time that Karyn had first disguised herself as a pegasus pony and been unable to fly.

“No, of course you wouldn’t. Pegasi fly by magic, not like birds who fly by aerodynamics. If changelings could appropriate magic that isn’t theirs, they would have taken over a long time ago.”

Derpy was confused. “But changelings can fly.”

“And so can bees and wasps. That’s more complicated aerodynamics, but it’s still not magical.”

Karyn and Derpy looked at each other. “How do you know so much?” asked Karyn. “Is that a unicorn thing? I mean, Twilight Sparkle knows everything about…everything, Lyra developed the inter-universe summoning spell while being a musician, and you’re a dentist, but you’re schooling us about the basics of magic.”

Now Colgate blushed. “It’s not all unicorns. But a lot of our parents make us go to school for a long time. An Earth pony wants their kids working as soon as possible, but unicorns apparently think you need to know how pegasi fly in order to fix their teeth.”

For Karyn, it was interesting to learn about unicorn culture, though she was surprised that Derpy didn’t know more. Perhaps, for all their friendship, the three types of pony still had different lives.

“What we could try,” Colgate continued, “is if you tried to become a unicorn, if I could piggyback my magic through you.”

Derpy asked, “What does that mean?”

Again she assumed her instructing tone. “When young unicorns have trouble mastering something like levitation, a parent or teacher will piggyback the spell with them.”

“I actually understand that, maybe,” said Karyn. “It’s similar to some of the techniques we use in IT.”

She had never attempted a unicorn transformation before, and she felt that Colgate might get offended if she copied her, not being quite as close a friend as Derpy was. Instead, she took the opportunity to attempt a hybrid transformation. She thought of the first three unicorns she could—Lyra, Rarity, and Twilight Sparkle.

What resulted was a pale-green unicorn with long eyelashes, a wide grin, and bangs over her horn. If a pony knew how it was made, and had met the three unicorns used, the derivations would have been clear, but to a stranger, it was a never-before-seen unicorn.

But as before, as soon as she tried to cast a spell, all she got was a headache. She wasn’t even sure if she was tensing the right muscles.

“Let me try to use your horn to cast a levitation spell,” said Colgate.

“All right.”

They stood next to each other. Colgate’s minty glow was reflected in Karyn’s horn, and they focused on a lamp on Derpy’s table. All of a sudden, the connection broke, and Karyn was back to her untransformed self.

“What happened?” she asked.

“That was very interesting,” said Colgate.

Derpy walked over and checked Karyn’s face. “Are you all right? Is it like before with the key?”

“No. I don’t hurt, but I’ve never been forced out of a disguise before. Let’s try again.”

The unicorn reappeared, but Colgate said, “Actually I have to go. Bye!” She raced out the door.

“That was weird.”

“It was.” Derpy trotted to the door and looked out through the fanlight. “I wonder if something happened to make her leave. I’m going to keep an eye on her. Meanwhile, want to keep practicing?”

“Actually, I find that I’m tired again. Why is it that when you sleep late, you want to nap right away?”

“I don’t know, but I’ll take you home. Next time, I’m sure you’ll be more energetic.”

Author's Notes:

Speaking of next time, here's the preview!

Derpy nodded, and then said, “Well, since we’re not going to worry, I can reveal the really good news.”

“Good news? Let’s hear it.”

“Well, I’ve got it all set up with everypony. We are going to Cloudsdale!”

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

“It looks good.”

“See, where this works is that I can wear a light sundress like this in the cold, and I can actually wear a warm pair of pants and a shirt.”

“Clever,” said Derpy, with a hint of conspiracy in her voice.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

“They never put these things in alphabetical order. It’s impossible to find on the first look.”

Derpy whispered to her. “It should be right after mine. Hooves, then Hubert.”

A little fake-out there, but next week will still be awesome!