Is the core of a video game a story or a gameplay system? Out of this scholastic debate we try to show in this paper that the main difference between games and other types of media is the player's engagement. We consider two radically different games, an interactive fiction designed by the student's of ENJMIN, "Fear Window" and a formal probabilistic game based on the finite drunkard's walk process "Drinking around Crocodiles". We try to show that, in both cases, the core of the game design and, as a consequence, the aesthetic of the game is the way to let the player feel his responsibility in the progress of the game.