Language has rules (grammar), a sound system (phonology), and a vocabulary (lexicon). When people share the same language, they understand each other as part of the same speech community by sharing the same language system rules. Language is acquired during childhood from the community as one's mother tongue or first language, while additional languages can be learned later through formal study or informal learning. Language contact and change can result in new language forms as the human capacity for language allows people speaking different languages to still communicate to some degree.