Philip B. Tan and Jason Begy teach this session from MIT's CMS.608 Game Design course, Fall 2010, starting with the basic question of what a game is and is not. They define the terms mechanic and dynamic, the building blocks designers use to describe how a game's rules produce actual play behavior. The lecture shows how identifying a game's core mechanic and dynamic lets designers focus playtesting and revision instead of tweaking everything at once, tying the discussion to Brenda Brathwaite and Ian Schreiber's Challenges for Game Designers and to Eric Zimmerman's essay on iterative design as a research process. The session functions as a working method for the rest of the course, treating playtesting as a structured, repeatable tool for improving a design rather than a one time check before release.