Rainbow Dash was left in charge to keep an eye on the growth/shrink ray machine that Princess Celestia and Luna had planned to use later that day. She was told to grow an apple from outside.
"Since I'm the only one here in Canterlot, I have to grow an apple from here. Celestia and Luna are on a trip to Las Pegasus, and it's my job to keep everything in control." Rainbow Dash said to herself. She then started to turn on the machine by pressing the growth ray button, and then aimed at the apple that was very down from her. However, when it touched the apple, the apple itself was unable to grow for some reason. "That's weird. How come it didn't grow?" Rainbow Dash wondered.
Not just far away from the castle, Rarity was planning to see Rainbow Dash there. Rainbow didn't even knew Rarity was coming, and she pressed the button again to make sure the apple could grow. However, when the beam was coming, it didn't hit the apple, but instead it Rarity, causing her to turn into a giant. She was a little bigger than the castle by height.
"Rarity? What are you doing here?" Rainbow Dash asked her.
"Oh, I just had to come visit you here at this place, and for some reason, your growth ray have made me grow. I didn't even pay attention to that thing of yours." Rarity replied to Rainbow's question.
"This isn't mine. It's Celestia and Luna's machine. I was supposed to grow an apple from far away from me, but somehow, you got in the way." Rainbow Dash told her.
"I can thank you for letting me grow. Now everyone else can call me "The World's Biggest Unicorn of Canterlot". "Rarity said, looking glad for being gigantic. She then started to walk away from the castle, causing the ground to shake a little.
"I don't think Celestia and Luna are gonna be happy with this…" Rainbow Dash said to herself again, this time, worried.