After the Tirek Battle...
The Main Six, now joined by the 3 Princesses, Discord, and Spike, were walking down a hallway of the newly minted Friendship Castle Playset. Twilight hung back so she could talk with Pinkie Pie.
"So...," Twilight began awkwardly, "you're actually a Changeling?"
"Yessiree!" Pinkie Pie responded cheerfully.
Twilight didn't know how to respond. Sure, it'd been weird when Tirek brought out her friends as bargaining chips, only for them to see that there was a Changeling in the mix, but it'd been a long and taxing day for everyone, and Twilight had somewhat become accustomed to Pinkie Pie's unusual level of randomness. In fact, this might actually help explain the more... Nonsensical things that Pinkie can do — the very dexterous mane with storage space, the squash-and-stretch shenanigans, the teleporting...
Although, thinking back on Pinkie's antics, Twilight DID remember one incident that couldn't be accounted for.
"So, if you've been a Changeling this entire time," Twilight began, "then why didn't you get blasted out of Canterlot by my brother's shield?"
"Oh, Twilight," Pinkie Pie laughed, "Shining Armor and Cadence didn't intend to blast me out, so I was unaffected!"
"Ah, that'd be Gusty's 'Law of Intention Exceptions'. I hadn't thought of that." Twilight relented.
Then she got an idea.
"You know," Twilight began, her researcher mode beginning to take hold, "this could give us an opportunity to learn more about the —"
Pinkie Pie put her hoof on Twilight's mouth and giggled. "Alright, you can run tests after this is over."
Twilight pumped her hoof in excitement, letting out a triumphant "Yes!!".
"Though right now, I think Celestia wants to talk to you," the Changeling pointed her hoof towards the white alicorn.
Twilight looked forward at Celestia, who was staring back at them with a mixture of confusion and patience, then said, "Yeah, it seems you're right." She quickly went to a trot to catch up with her former teacher, leaving Pinkie smiling.
This had certainly gone over better than she'd expected.
A while later...
Pinkie was now sat in the library of Twilight's new home, waiting while Twilight got everything set up.
"The Changeling Experiments, Session 1," Twilight spoke into a tape recorder. "Date: August 10. Test 1: Disguise Disruptors."
Twilight turned to Pinkie Pie. "Alright, is there anything that you've discovered that disrupts your form? Anything that forces you to drop your disguise?"
Pinkie Pie thought about this for a moment. "Well, there was this one time where I ate oatmeal, got sick, and while I was throwing up, my disguise dropped. The throwing up thing is likely universal, though the oatmeal part's probably just a me thing."
Twilight jotted down some notes. 'That at least explains the Oatmeal story,' she thought to herself, then asked, "Anything else?"
"Not that I can think of off the top of my head," Pinkie admitted, poking the top of her head.
"Then may I run a few tests to see what exactly triggers a drop in disguise?"
"I thought you were already—"
Before the pink pony could finish her statement, a torrent of cold water was dumped onto her head. The shock of suddenly becoming wet, along with the sheer coldness, caused Pinkie's disguise to drop.
"I thought so," Twilight noted. "It seems that unexpected stimuli can cause a transformation."
Twilight then floated a towel towards Pinkie. Pinkie, however, was already doing her usual thing: being completely random by twisting her tail tightly, then letting it rip, using it as a fan to blow-dry herself off. Twilight shrugged and set the towel back on the counter.
"Test 2: Squash-and-Stretch."
"Alright, now let's test your powers of single-form rubber hose manipulation."
Pinkie nodded, then stated bluntly, "I have no idea what that means!"
Twilight glanced up at her amusedly. "It means I want to see what you can do to your forms without actually changing between them. Like how you sometimes stretch your body to emphasize movements?"
Pinkie Pie zoomed rather cartoonishly up to Twilight, her body stretching as if it were rubber hose. "Still no idea what you're talking about."
"Like what you just did there! Y'know, the... the cartoon-y stuff?"
"Oh, that? Yeah, that's not actually related to my Changeling Powers."
Twilight did a double-take. "Wait, WHAT?"
"I mean, I suppose I could do it like that, but that's not how I do it."
Twilight barely managed to stammer out, "B-but how!?"
Pinkie laughed in response. "I can't tell you, silly; that wouldn't be any fun!"
Twilight fainted from exasperation.
"Test 3: Disguised Magic."
Twilight turned to say something to Pinkie, but stopped when she saw that the party planner was no longer in her chair.
"Pinkie? Where'd you go?" Twilight asked cautiously.
The sudden answer came from behind the purple Princess, startling her: "I'm right here, silly!"
Twilight turned to see Pinkie lying across the counter and playing with Gummy, who'd appeared on her lap.
"What– when– where– why?" Was all that the book horse could manage.
"Because it's playtime for Gummy!" she replied.
Twilight could only roll her eyes and smile as Pinkie played with her beloved pet. "I suppose we can be done for the day."