The potential of video games for learning is now widely accepted among the community of Educational Technology. However, there is a critical lack of guidance for the design of educational games. In order to provide such guidance, there is a need for a solid theoretical basis regarding the nature of learning in games. This paper redefines what a game is, in semiotic terms, enabling four groups of strategies to be formally identified, depending on how the knowledge to be acquired is inserted into the game. These four groups are: systemic learning, when knowledge is embedded in the game mechanics; winner strategies, when the game provides an environment in which knowledge is required to reach the game's goal; loose coupling, when knowledge is arbitrary required to unblock the progression towards the game's goal; and contextual coupling, in which the game serves as a context for the exposition of static learning material. This theory is then put into practice by analyzing three commercial educational games. It constitutes a first step towards Instructional Game Design.