When Twilight stepped into her mentor's bathroom, she saw a giant alicorn covered with roaring flames.

Twilight screamed.

The flaming creature cannonballed into the bathtub. Surrounded by steam, Celestia said, "Don't worry. It's just today's lesson."

"A lesson in what?"

"Alicorn genealogy. Long ago, we needed somepony who could create and control a sun at will."

"Create?"

"You don't think suns just happen, do you? But unicorn magic wasn't strong enough. An alicorn princess knew we needed more firepower than unicorn magic alone could provide. So she married a kirin."

"Ok."

"Their daughter was half kirin/nirik, half alicorn. So she married another kirin. THEIR daughter was two halves kirin/nirik, one half alicorn."

"I don't think that's how math--"

"Generations later, the combined powers of many halves can create and control a sun at will!" Celestia preened herself. "And that's how we have sunlight whenever we need it today!"

Author's Note

Obviously takes a bit of inspiration from "Twilight Takes a DNA Test," for which it suggests extra backstory.