1564 years ago...
"Now, it is very important that you measure correctly, or the entire potion would be ruined." Starswirl the Bearded said, never taking his eyes off the beaker in front of him. "Luna, be a dear and get me three sprigs of heart's desire from the herb cabinet, will you?
"Yes Sir!" The young filly exclaimed, hopping down from the stool and trotting merrily over to the large green cabinet on the other side of the room. Meanwhile, Starswirl turned to the other occupant of the room. He picked up the heavy potion book he was reading from and slammed it down on the table with a loud crash.
"WHO WHAT WHERE?" Celestia cried as she woke up from the noise. Starswirl laughed at her and turned back to the potion to make sure it wouldn't boil over.
"Celestia, pay attention. If you are to rule this country one day you must take your studies seriously."
"Whatever. When I'm in charge I'll have ponies do this stuff for me." Celestia waved a hoof and set her head back down on the table to resume her nap. " Starswirl's horn glowed and a small spark flew from it, hitting Celestia in the flank.
"Faust Dammit!" She yelled. She grumbled about how Starswirl would spend time in the dungeon once she got her hooves on that crown.
"Language, Celestia. Luna, can you keep an eye on this while I prepare the next step?"
"I sure can!" The filly beamed, taking the glass stirring rod in her mouth and continually stirring the potion.
"Suck up." Celestia mumbled under her breath. "One of these days you'll find yourself waking up on the moon, you little brat."
"I heard that, Tia." Luna responded. "You're just jealous because Professor Starswirl like me more."
"I like you both equally, but Celestia needs an attitude adjustment." Celestia rolled her eyes in response. "By the way, did you do something to your mane?"
"What? No I-" Celestia reached up when she realized her long bright pink mane wasn't resting on her shoulders. "STARSWIRL WHAT THE BUCK DID YOU DO TO MY MANE?!" She cried as the ethreal flowing rainbow of colors caught her eye.
"A princess needs to look beautiful." He said, Hoof covering his grin as he tried to hold back his laughter. Luna dropped the rod and fell out of her stool from her laughter.
Modern times
"And I never figured out how to reverse the spell." Celestia said, sipping her tea. Twilight sat in confusion, wanting to laugh but being afraid to at the same time. That also explains the skeleton hanging in the dungeon with the robes and the words etched into the wall: "Totally worth it"