"The sun's gonna die some day!"
Celestia finds out that the sun will die but the moon won't. Grabbing her awesome sidekick, Twilight Sparkle, she heads out to stop Luna this catastrophe from happening.
(AU tag added as OOC speech/thoughts will be shown. It's also non-canon.)
Princess Celestia finds out that the sun will die someday. You know what happens next.
After an argument breaks out, Celestia tells her sister, Luna, as a prank that the moon is now made out of cheese. Of course, Luna goes to see for herself. What she doesn't realise that getting to the moon isn't really that easy.