The Growth Potion is a unique object unlocked at the penultimate main goal of the game. Emperor Xizzle gives your sim the potion, and instructs them to drink it on the Sun Deck in order to destroy the antenna that Optimum Alfred had put there as part of his scheme to take over Strangetown. Upon drinking the potion, your sim will grow to around three times their size and stomp on the antenna, destroying it.