His explanation (slightly grammar-corrected by myself):

“Rarity was practicing a spell taught to her by Cupix, which involves enabling her to be invulnerable to pain. As she nearly got the hang of it, it went wrong, thus resulting in her bottom increasing greatly in size and her intestines becoming all twisted, thus giving her massive gas problems, resulting in her letting loose massive farts. Cupix came to her rescue by comforting her with a bum massage while he harnesses his energy to return her bottom to its former fabulous glory.”

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(Alternate description 1: )

Dear Princess Celestia,

Today I learned that you should be very careful while practicing invulnerability magic. While it can be powerful when used correctly, it can sometimes backfire, resulting in random butt inflation. And Pinkie Pie trying to tie a string to your hoof and call you a balloon. Pinkie Pie’s been banned from Rarity’s store for the next week.

Your faithful student, Twilight Sparkle Inflatorpill

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