Because Prankster People Prefer Pranking Ponies
It was a dull day in Ponyville, especially since it was raining, and if you walked outside all you'd see is ponies running...er, galloping, to their homes for shelter. This was a price to pay for the beauty of spring, however, and most ponies realized it was a good tradeoff. Some ponies, like Twilight Sparkle, used the time for reading indoors. Despite her royal position, Twilight was allowed a vacation to Ponyville every 2 weeks, which she loved! Being a princess was hard work, even for Twilight, and some time to relax with her best friends was always welcome. Currently she was reading a novel, Unicorns First Rule, by Terry Goodkind, one of Twilight's favorite authors. She was about halfway through the book when she heard a crash coming from her bedroom. Bookmarking her page, she walked up the stairs to her room, expecting Rainbow Dash with a goofy smile, apologizing and saying it was a trick gone wrong that caused Twilight's insurance rates to go up. She slowly opened the door to her bedroom, seeing a pink earth pony shakily stepping off of a crazy flying....thing. Pinkie didn't seem to notice the fact she broke the roof as she hopped over to Twilight and gave her a Pinkie hug.
"Hey Twilight! Watcha doin'?" Pinkie asked innocently. Twilight couldn't bring herself to be mad at Pinkie, as was often the case, so she smiled at Pinkie instead.
"Hello Pinkie Pie, I'm glad to see you. I was just reading actually. Would you like something to drink?"
"Sure, thanks!" Pinkie said, grinning and going downstairs. Twilight took this opportunity to clean up the mess Pinkie made, using magic to fix the hole in the roof, and move Pinkie's...machine safely onto the front lawn. She then followed Pinkie downstairs.
"So Pinkie, what brings you here on this rather rainy day?" Twilight asked as she poured tea for herself and Pinkie.
"Dunno, I just felt like this is where I need to be today" Pinkie replied thoughtfully. This was met with a look of mixed confusion and humor from Twilight. As soon as she thought she was used to Pinkie's antics, Pinkie did something wacky like crash through her roof, even though she didn't know why she came in the first place.
"So Twilight, practice any cool spells lately?" Pinkie asked in an attempt to clear the awkward silence caused by Twilight's thoughts. Twilight re-focused on Pinkie.
"I haven't had much time for practicing new spells, though I did find one on inter dimensional travel"
"Really? Oooh, what is inter dimens...er, whatever you just said?"
"Well, basically" Twilight explained, "The spell allows you to go to another world. It was the only major spell Starswirl ever worked on where he utilized the help of another pony. Hoofstein's theory of relativity was based on some basic principles that Starswirl discovered while creating the spell"
"Did they ever get it to work?" Pinkie asked, intrigued.
"No pony knows" Twilight said, "But I intend to figure it out!" She said, determined.
"Oh! I know! You could use it on me! Then I could see if it works!" Pinkie exclaimed excitedly.
"It doesn't quite work that way, Pinkie. As opposed to teleporting one pony into a different dimension, Starswirl intended for the spell to open up a portal that lasted for one hour, or until some pony went through it. And if you go through, I'm not certain you could get back" Twilight explained.
"That makes sense, I suppose. But what if we opened the portal and waiting for something to come out of it?"
"Actually Pinkie, that's a good idea. Let's do it" Twilight said happily. Little did they know, that portal would cause a chain of events that would sort of change part of their lives, but have no impact on Equestria as a whole whatsoever.