Celestia had heard of Twilight Sparkle first through Cadence's ravings about the young filly she babysat. The younger princess's tales had intrigued her, the filly described seeming to fit the prophecy perfectly.
Discovering Twilight was the perfect candidate, Celestia found herself with another block in the road. Almost all of her free time was spent teaching Cadence how to act as a princess. Eager to trade one student for another the manipulative Celestia comes up with another plan.