Princesses Celestia and Luna, while visiting in the Crystal Empire, insist Cadance and Shining Armor take a night off while they watch Flurry Heart.
When Flurry asks Luna to play with her, she is taught how to play an ancient game from the princess' own fillyhood. Simple, harmless fun, right?
....Right?
(Note: Flurry Heart is 2 in this story)