"W-what are you doing?" Twilight whimpered, backing away.
"Don't worry, we'll only leave you for a few minutes." Dash replied, as she and Derpy advanced on Twilight. After a couple of minutes, they had backed her up against the wall and taped her there.
"Let me down!" Twilight demanded.
"We will, we will," Dash replied, "but first I want to try something."
She placed a piece of tape over Twilight's eyes and mouth, and stepped back. Twilight waved her hooves wildly in an attempt to free herself, before Dash decided to tape her legs to the wall as well.
"This is going to be the greatest prank ever!" Dash squealed happily. She flew over to a table and placed the books that had been on it onto the floor. She then picked up the table.
She held it to the ceiling upside-down and signaled to Derpy, who flew up and taped it in place.
"Now the lamp!" Dash whispered, glancing over at Twilight cautiously, checking to make sure that she couldn't hear them. Her ears were twitching, but never angled towards where Derpy was now taping the lamp to the table.
Dash picked up the stack of books that had originally been on top up the table, and taped the whole stack there, right next to the lamp where they had originally been. Dash looked around the room before whispering in Derpy's ear.
"Derpy, you finish down here. I'll do the upstairs."
Derpy got to work inverting the bookshelves as Dash flew upstairs to Twilight's bedroom. Derpy was hard at work flipping all the books on the shelves upside down, when Twilight finally decided to intervene.
"Mhmmhfmm!" she mumbled through her duct tape gag.
Derpy turned around at the sound, her eyes losing focus and traveling to the corners of her eyes. "What was that, Twi?"
"Mfh! Hmfhmm!" she said again.
Derpy walked over to her and removed the duct tape from Twilight's mouth. "Huh?"
"I said, what are you doing to the library! And where did Rainbow Dash go?"
Unable to come up with a reasonable excuse, Derpy stumbled with words before replying. "I don't want to spoil the surprise." Was all that was said, after a moment of hesitation.
Twilight asked again where Rainbow Dash had gone to, pointing out that even though she couldn't see through duct tape, she didn't hear Dash trying to come up with any excuses.
"She went outside to look for more duct tape." Derpy replied.
Twilight found nothing wrong with Derpy's statement. "Okay, I believe you." Twilight said.
Derpy went back to inverting the furniture, being sure not to make any noise.
Suddenly, a crash came from Twilight's bedroom.
"What was that?" Twilight asked, worried that it might have been something valuable.
Derpy flew up to the top of the stairs before replying, covering for Dash. "My bad!" she called down.
"What fell? And did it break?" Twilight repeated.
Derpy looked into Twilight's bedroom to see that Dash had finished with everything except a vase, which had fallen and shattered on impact. "It's just a vase." Derpy answered. "I'm sorry I broke it..."
"Now the more important question. What were you doing in my bedroom?"
"Just working on more of the prank." Derpy replied.
"Okay. And don't worry, the vase wasn't expensive. Just clean it all up."
"Okay!" Derpy turned to Rainbow Dash, who had not said anything during the entire exchange, listening instead. "Thanks for covering for me." she whispered to Derpy.
"It's fine. You still have to pay for it though. You heard our whole conversation right?" Derpy whispered back.
"Yeah."
"So you're going to fly out the window and come in the front door with a bunch of duct tape, right?"
Dash nodded, flying silently out the bedroom window, which Derpy closed carefully before going back downstairs. "Sorry Twilight." Derpy said. "I'll pay to replace it."
"It's fine, just try to be careful."
"Okay. The prank is almost ready anyways, so I'll let you down soon."
"Thank goodness." Twilight sighed. "My hooves are stiff, and it's getting frustrating not being able to see what's going on around me."
"I'm back!" Dash called, walking in the front door nochalantly.
"Oh, there you are." Twilight said. "I was wondering where you'd gone."
"Just to get more duct tape." Derpy and Rainbow Dash went back to turning the library upside down, Dash working on the furniture while Derpy turned all of the books on the bookshelves upside down.
"Done!" Derpy called, after she and Dash had finished.
Dash flew over to Twilight and removed the tape from her hooves, and led her to the middle of the room. "All right, you can look now!"
Twilight removed the blindfold with her recently freed hoof and gasped. "No...bucking...way." She collapsed onto the floor in a fit of laughter. "Ooh, you two got me, that's for sure. The good news is, once the duct tape disappears, I can just put everything back with magic."
Derpy looked confused. "What do you mean, once it's gone?"
"Anything transferred between worlds disappears when the fourth wall is repaired."
"Why can't you just remove it with your magic now?"
"There's no magic in the world where duct tape comes from, so I can't use magic on it." she said simply, flipping all of the books on her bookshelf right-side up again. "Now I just need to put an anti-gravity spell on everything so that nothing breaks or falls when the tape disappears."
"You do your magic, then we should go around and see how Ponyville is faring with the duct tape." Dash decided.
"I have to deliver more mail!" Derpy giggled, having completely forgotten. She picked up her mail bags and flew out the door.
Twilight cast her spell and walked outside with Rainbow, her eyes widening as she saw the utter chaos that ensued.