Each culture dictates rules for a certain, expected behavior and shares values and beliefs. Enculturation is the natural and unconscious process by which we acquire our own native culture. The United States is a melting pot of hundreds of different ethnic groups, each in itself a distinct culture. Most immigrants choose their relationship with their native and their newly acquired culture. It often happens that through assimilation the pure native culture is slowly lost, thus merging the old and the new culture and making them, after a period of time, indistinguishable. When immigrants experience acculturation, they adopt a second set of cultural rules which can coexist with the rules of their native culture, replace the rules of the old culture, or modify the old rules so that they complement the new ones. Not too many immigrants are truly bi-cultural and maintain and use two cultures simultaneously with equal intensity. A relatively recent phenomenon is retro-acculturation, the search for ethnic identity or roots by second, third, and fourth generation Americans who have lost most of their cultural traits, including fluency in their ancestors' language.