twidash1993 put the potato there.

Rarity sprung awake from her Fainting Couch™, gasping for air. It was a peculiar dream that she'd had, consisting entirely of Twilight Sparkle making furiously playing music with a plastic straw and a cup of McDonalds Sprite.

Also, she apparently had a potato on her horn.

Rarity, I mean.

Sweetie Belle, the not-a-giraffe one, wandered into the room. She immediately spotted the potato on her sister's horn.

"Ah, it's time for the harvest!"

Rarity shrunk back from the remark. "Harvest? Whatever do you mea--"

Sweetie plucked the potato off Rarity's horn and trot off to the kitchen, where she proceeded to immediately light said potato on fire.