The doorbell to Sugarcube Corner jingled, and a stallion walked in. He was unfamiliar to most ponies in the building; he'd visited town once before, but only for a day or two. His name was Obsessive Compulsive.

Pinkie jumped up when she heard the bell. "OC! It's good to see you again!"

He smiled. "It's nice to see you too, Pinkie. I was heading to Princess Twilight's castle on my most recent investigation, but I couldn't resist stopping by to say hello... and get a snack."

"Of course! What can I get for you?"

"I've heard you like to experiment. I'll take whatever you've been working on."

Pinkie beamed. "That's great! I'm really excited about this one. I coated a cupcake liner in chocolate and let it dry, and then I had a cupcake liner made of chocolate! Kinda like a bread bowl, but made of chocolate and for cupcakes! I've got a batch of cupcakes using the chocolate liner in the oven right now, and they should be out in just a few minutes! Just enough time for us to catch up."

"Wait, did you say they're in-"

"So, what's bothering you this time?"

He was sure she knew what she was doing, so he shrugged and moved on. "I'm just curious about this 'mirror portal' I've been hearing about. Trashy newspapers keep insisting she has a secret portal that takes her to other worlds. I want to find out if that's true or not, and if it's true, how it works. This could be a huge breakthrough! Nopony's been able to figure out dimensional transport since Starswirl the Bearded, and he sure wasn't leaving any notes. I tried to ask him, but he refused to see me! I still don't even know how he was returned to present day!"

Pinkie sighed. "He can be a bit of a grump. I'll see what I can do to get him to talk to you. As for your other question, though, yes!"

"Yes, what?" he asked, having lost track of the conversation.

"Yes, she does have a mirror portal! It only goes to one world, though."

"Well, that's a bit of a disappointment. Do you know if there's any way it could be changed to go to other worlds?"

"Well, a while back there was another one that went to a different world, so probably more could be made. Funny you mention Starswirl, actually! He made both portals!"

He sat up straighter. "Oh, that's nice."

"What is?"

"Well, it just fits so nicely and makes so much sense! My cutie mark is in finding out how things fit and make sense, so hearing that is incredibly satisfying."

"Gotcha! Anyway, sometimes she powers the portal with a magic book that connects to another magic book on the other side of the portal, so maybe if somepony wanted to make another portal they could connect a magic book like that to a mirror and then it would open up to where the other book is! But I don't really know, that's more Twilight's thing."

"Uh, Pinkie?"

"I bet she could tell you all about how it works, with all the fancy magical words for it!"

"Pinkie?"

"While she's at it, she can tell you how we brought back the Pillars!"

"Pinkie!"

"What?"

"Why does it smell like burning chocolate?"

Pinkie gasped. "Uh oh. Be right back!" She darted into the kitchen and pulled a rack out of the oven. Even from a distance, OC could see the smoke coming off of them. Pinkie returned. "So, it turns out actually baking the cupcakes in chocolate wrappers wasn't a great idea. They melted, and now the cupcakes are stuck to the edges of the pan and chocolate is stuck on the bottom and the whole thing is a mess." She sighed and wilted a bit. "I guess chocolate wrappers are only for cooled cupcakes. Ooh! Or cold cupcakes! Like an ice cream cake, but a cupcake, and then with a chocolate wrapper! Wouldn't that be delicious?"

"It certainly would be," OC agreed.

"So... would you like a normal cupcake instead?"

"Yes, please."

Author's Note

Requested by Viper Pit

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I'm planning to do a crossover story with Infinity War titled Elements of Infinity. Though I haven't seen the movie yet, I have a few things planned out. One of them is that the ponies will use a mirror portal to get to earth, using a magic book, just like Pinkie mentions here. If you want to see that story when it comes out, follow me! It'll probably be the next thing I write (unless I finish my third Familiars story first).
Shameless plug complete