Chomsky's theory proposes that children are born with an innate Language Acquisition Device containing Universal Grammar that allows them to learn the structure of any language. It is a nativist theory which assumes children have internalized knowledge of language that allows them to creatively use new words. A key aspect is the existence of a critical period for language learning during which children can acquire language spontaneously without formal teaching. The case of "Genie" provided an example of a child deprived of language input, but did not conclusively prove the theory due to uncertainties over her mental state and whether the critical period had passed.