All games share four defining traits

No matter the platform, the games share some essential features. May be "Tic Tac Toe" or "Gears of War 3" you can find some similarities between them. Let's see four main points:

•The goal: is the specific outcome that players will work to achieve

•The rules: place limitations on how players can achieve the goal.

•The feedback system: tells players how close they are to achieving the goal.

•The voluntary participation: requires that everyone who is playing the game knowingly and willingly accepts the goal, the rules, and the feedback.

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Source: McGONIGAL, Jane. The reality is broken. London: The Penguin Press, 2011, pg. 21