In learning languages, a distinction is usually made between mother tongues, second languages, and foreign languages. A mother tongue is the first language or languages one learns (or acquires) as a child. When immigrants come to a new country and learn the language of that country, they are learning a second language. On the other hand, when English-speaking students in the United States learn French or Spanish in school, or when Brazilians study English in Brazil, they are learning a foreign language. The acronyms ESL and EFL stand for the learning of English as a Second and as a Foreign Language. Many theories about the learning and teaching of languages have been proposed. These theories, normally influenced by developments in the fields of linguistics and psychology, have inspired many approaches to the teaching of second and foreign languages. The study of these theories and how they influence language teaching methodology today is called applied linguistics.