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In John Carter, Carter is abducted by the Tharks and is made to stay with the hatchlings.
He drinks something (water? milk?) and then gets dizzy and starts understanding what the Martians are saying.
What caused this?
In the film, a thark (I think it was Sola but I'm not 100% sure) calls the milk "the Voice of Barsoom." Presumably it taught him the language.
Required, but never shown