For example, the LAD already contains the concept of verb tense and so by listening to word forms such as "worked" or "played". The child will then form a hypothesis that the past tense of verbs are formed by adding the sound /d/,/t/ or /id/ to the base form. Yang (2006) also believes that children also initially possess, then subsequently develop, an innate understanding or hypothesis about grammar regardless of where they are raised.[7] According to Chomsky, infants acquire grammar because it is a universal property of language, an inborn development, and has coined these fundamental grammatical ideas that all humans have as universal grammar (UG). Children under the age of three usually don't speak in full sentences and instead say things like "want cookie" but yet you would still not hear them say things like "want my" or "I cookie" because statements like this would break the syntactic structure of the phrase, a component of universal grammar. Another argument of the nativist or innate theory is that there is a critical period for language acquisition, which is a time frame during which environmental exposure is needed to stimulate an innate trait. Linguist Eric Lenneberg in 1964 postulated that the critical period of language acquisition ends around the age of 12 years. He believed that if no language was learned before then, it could never be learned in a normal and functional sense. It was termed the critical period hypothesis and since then there has been a few case examples of individuals being subject to such circumstances such as the girl known as Genie who was imposed to an abusive environment, which didn't allow her to develop language skills.