Cohesion between content and gameplay can lead to games that are educational and enjoyable at the same time, making them effective. Endogenous designs, wherein the game elements emerge from the content, lead to such cohesion. However, endogenous design is not commonly observed in practice. Lack of guidance on conceptualizing endogenous design is an important factor hampering production of effective educational games. In this paper, we present ‘Endogen’ framework for designing endogenous educational games. The framework synthesized using the ‘function-behavior-structure’ approach is composed of a set of design strategies that we discovered in our recent work. It is validated using a multi-method approach wherein designers use the framework to design games and then rate it on various parameters. Results indicate that Endogen achieves the stated objective of creating endogenous design, therefore offering exciting content integration possibilities for game designers.