The day started out like any day. Rarity got up, fed her cat, woke up her little sister, and while her sister was at school, worked on a fashion design. It was when Rarity picked up Sweetie Belle to go home did it happen. She walked to her school to see her sister waiting in front. Sweetie automatically ran up and said, "What took you so long?"
"I'm sorry, Sweetie. I was very busy." But just then, two fillies, a pink filly and a silver filly, walked up to Sweetie Belle.
"They must be those two mean fillies Sweetie tells me about. Just look at the expressions on their faces!" And of course, they did both have a smirk on their faces.
"Hi there, Sweetie! I hope to see you tomorrow!" the pink filly said, who Rarity presumed to be Diamond Tiara due to her cutie mark.
"Go away, Diamond and Silver." Sweetie groaned. Diamond then looked up at Rarity.
"By the way, is that your big sister or your mother, because she looks to old to be your big sister!" She said as she and her friend sashayed away laughing. Rarity watched them in horror. Sweetie noticed and said,
"Don't take it personally. They always act like that." But Rarity still didn't move. She then turned and trotted in one direction.
"Come on, Sweetie." She said.
"But home is that way!" Sweetie said,pointing her hoof in the opposite direction.
"We are not going home. We are going to Twilight's, to see if she has anything to make me younger." Rarity said.
"But you're only 25!" Sweetie cried as they walked towards Twilight's house.
"Your classmates are right, even at that age I'm getting old. And look, we are already here!" They went to the door of Twilight's castle and knocked. Twilight opened the door and welcomed her friends in.
"What can I do for you?" She asked.
"Twilight, I need something that will make me younger."
"But you're only 25."
"I know, but a filly today called me old."
"Was that pony, by any chance, Diamond Tiara?"
"I think so." Twilight sighed.
"If you insist." She then went to her cabinet and pulled out a sack of red berries. "These are youth berries. With everyone you take, you will become younger by one year. It affects you in thirty seconds from taking it." Rarity grabbed the bag quickly and ran home with Sweetie. She hurriedly went to her table and opened the bag.
"Are you sure this is a good idea?" 
Sweetie asked. Without responding, Rarity quickly ate one. Then she ate another. And another. And another. Soon, she had quickly eaten all the pills. She looked at Sweetie and said, "How many pills were there?"
"20."
"And I'm 25, which means, oh no...." It had already been 20 seconds! 10...9....8.....7.......6.......5......4......3......2.....1! Then all of a sudden, she began to shrink, and as she shrunk, her cutie mark vanished from her flank and her features became more foal like. And when it was over, Sweetie saw not the older sister she knew, but a filly that was exactly her age.
Sweetie stared in shock at her once big sister. Rarity looked into Sweetie's eyes and said, "Sweetie," But then noticed her voice was changed into a squeak. "What's happened to me?" She asked in shock.
"You're a 5 year old filly Rarity." Sweetie said. "You're only my age now. "
"NO!" She yelled, but then noticed her flank! "And my cutie mark is gone!"
"Must have been a side effect."
"But how shall I fix this?" Sweetie told her the only thing that was true.
"I don't know. Twilight knew how to make younger, maybe she can make you older." Sweetie said.
"Of course!" Rarity said. So they went down the path to Twilight's castle, and when they reached there, they knocked on the door. Twilight opened the door and said,
"I can see that you are MUCH younger now."
"Twilight, I took to many youth berries and now I'm a five year old filly, and I don't know how to change back!" Rarity yelled.
"I thought this might happen. Did your cutie mark disappear?"
"Yes."
"Here's what you have to do: If you re-earn your cutie mark within 7 days, you will change back into a mare. If not, it won't work." Twilight said.
"What won't work?" Twilight turned to see Spike behind her!
"Spike! What are you doing here?" Twilight said. Spike looked at the mare turned filly and said,
"Is that Rarity? Twilight, did you do some of your voodoo on her?"
"No! Yes! Kind of! Yes, that is Rarity, she took some youth berries, and they had the wrong effect." Twilight said. Spike walked up to Rarity, and then Rarity was shocked to find out that she was now eye level with him!
"How can I get you to change back?" He asked.
"I need to earn my cutie mark back, but I only have 7 days!" Rarity said.
"Well, if that is what you need, I will help you!" He said as Rarity, Sweetie Belle, and he walked out the door. " So what's our first stop?" Spike asked.
"The mine. I need to earn it for dress making, and what makes a dress more bedazzling and sparkling than jewels?" Rarity said.
"Ok, I had better bring something to carry the jewels!" Spike said as he walked away and came back with a bucket. "I know it's not very big, but it will do, I think." He said. So they the three walked until they came upon the mine.