Good games offer players a set of challenging problems and then let them practice them until they have routinized their mastery. Then, the game throws a new class of problem at the player (this is sometimes called a "boss"), requiring them to rethink their taken-for-granted mastery. In turn, this new mastery is consolidated through repetition (with variation), only to be challenged again. This cycle of consolidation and challenge is the basis of the development of expertise in any domain.
— Literacy, Video Games, and Popular Culture
by James Paul Gee