By calling language an instinct, Pinker means that it is not a human invention in the sense that metalworking and even writing are. While only some human cultures possess these technologies, all cultures possess language. As further evidence for the universality of language, Pinker—mainly relying on the work of Derek Bickerton—notes that children spontaneously invent a consistent grammatical speech (a creole) even if they grow up among a mixed-culture population speaking an informal trade pidgin with no consistent rules. Deaf babies "babble" with their hands as others normally do with voice, and spontaneously invent sign languages with true grammar rather than a crude "me Tarzan, you Jane" pointing system. Language (speech) also develops in the absence of formal instruction or active attempts by parents to correct children's grammar. These signs suggest that rather than being a human invention, language is an innate human ability. Pinker also distinguishes language from humans' general reasoning ability, emphasizing that it is not simply a mark of advanced intelligence but rather a specialized "mental module". He distinguishes the linguist's notion of grammar, such as the placement of adjectives, from formal rules such as those in the American English writing style guide. He argues that because rules like "a preposition is not a proper word to end a sentence with" must be explicitly taught, they are irrelevant to actual communication and should be ignored.