A game is an activity, generally recreational in nature, involving one or more players. Most commonly, the word refers to board, card and video games as well as sports. A game generally consists of a goal that the players try to reach and a set of rules that determines what the players can or cannot do. Games are played primarily for entertainment or enjoyment, but may also serve as exercise or perform an educational, simulational or psychological role. Group leaders can use games for creating or altering an individual's ego-boundary or a group's interpersonal boundaries. Games can also be used to alter an individual's or a group's mood. Since games can generate a higher and less cognitive arousal level, they are useful after a large meal or a long and tedious task, but are not good for pre-sleep needs.