They escalate to stealing "retirement grease" from Springfield Elementary's kitchen.
Groundskeeper Willie catches them, leading to a memorable standoff where Homer tries to fake a Scottish accent, claiming to be from "North Kilt-town". The "Greasy" Conclusion [S10E1] Lard of the Dance
Homer realizes that fat can be sold for profit. He begins by frying massive amounts of bacon, only to realize the bacon costs more than the grease is worth. [S10E1] Lard of the Dance