The Powerpuff Girls (1998)

S01E15 Paste Makes Waste

Buttercup mocks her classmate Elmer, who has a penchant for eating paste. When a fly contaminated with atomic chemicals lands in the paste, Elmer transforms into a rampaging Paste Monster. To escape this messy predicament, Buttercup must do something that goes against her usual tough demeanor—she has to apologize.