First language is typically acquired without effort, simply by exposure to it in early life. Individuals can also learn and master a second language to which they are exposed in a family or environmental context with as much ease as their first language. However, there are circumstances in which where an individual's exposure to their first language drastically decreases.
For example, imagine an individual who is stranded in a deserted place, and who has had no contact with his/her mother tongue for many years. In this situation, would this individual forget his/her mother tongue completely? Or, would this person simply have difficulty in accessing certain elements or lexical items in the language? And if either of these outcomes were to occur, what causes this attrition? These questions could be baffling ones to be answered, since such a situation is hypothetical one; however, two or more languages' coming into contact is real and an observable common phenomenon and the results of this contact can be explored and understood.