"Core-uption" is an episode of Catscratch from Season 1.
When Human Kimberly gets an "F" on the science project for saying that the earth's core is made of rainbows and unicorns, Gordon helps her get an "A" by drilling down to the core and stuffing the project inside of it. This causes the earth to become a fairy wonderland and changes the appearance of everything.
Mr. Blik: My beautiful insides, gone! I'M A WALKING, TALKING BAG OF STUFFING, YOU LOSEPAPER!
Mr. Blik: Waffle, don't you pull that--! (pulls string) (numb) I love you. do you love me? (he starts getting stressed) AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH!!!!!!!!!!!!!! THIS IS A NIGHTMARE!
Human Kimberly: It's still an (crash) F!!!! (bawling)