Naturalization
Most people have the nationality of the country where they are born and grow up. If they move to another country they will still keep their own nationality at first, but if they stay there for many years and want to stay there for the rest of their lives they may want to change their nationality. Different countries have different laws about nationalization. Usually the person will have to make a promise in a court of law. He will have to promise to be a good citizen and obey the country's laws.