The Lucy Show

Lucy and Viv Take Up Chemistry (1x26)

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Viv regrets letting Lucy talk her into joining her for a night-school chemistry class. Lucy gets carried away trying to invent a youth serum and develops a huge ego between explosions. To teach her a lesson, Viv and the professor make her drink her own concoction, which acts as a sedative. When she awakens, she's horrified by the results of her youth formula.