Invader ZIM (2001)

Season 1: Episode 9

When it begins to rain one day, Zim becomes worried because he’s never experienced rain before. Dib takes notice of his fear and he coaxes Zim into stepping into the rain, only to discover that the Irken race has an innate weakness to water, which causes a burning sensation upon impact. Dib decides to exploit this new fact to create the ultimate weapon to stop Zim once and for all, but Zim himself attempts to find a way to protect himself from this new found weakness, which all leads up to a confrontation that neither of them would have expected.