Rainbow Dash. She's always going to the princess, and Twilight doesn't know this. It turns out Celestia is Rainbow Dash's mother, and they write to each other. This makes Twilight and also BFF Applejack confused. Why isn't RD a princess? Who is her sister? Was she at home for school? To figure it out, Twilight needs to look at a reflection made by a shiny-glass prism.