Hadi-Tabassum believes having a separate life context for each language she learned in childhood enabled her to switch easily among five different tongues. She states that the success of dual immersion bilingual programs is largely dependent on whether they immerse students in each of the involved languages separately and help students have a discrete context for the use of each language. It is essential that students and teachers work exclusively in Spanish or exclusively in English, for example, during certain times of day or for certain content areas. Although all subject matter should be taught at least in partially in both languages, class discussions, assignments, and materials--right down to daily agendas--should never mix the two. The author gives an overview of two different theories of how two languages are developed. The language independence theory states that each language develops and resides in its own separate area of the brain, whereas the language interdependence theory assumes two or more languages develop with a common underlying cognitive structure.