Game classification is the classification of games, forming a game taxonomy. Many different methods of classifying games exist. Physical education There are four basic approaches to classifying the games used in physical education: Game categories This is a classification scheme proposed by Nicols, who classifies games according to three major categories: the game's physical requirements (i.e. what the game requires in addition to the players — equipment, size and nature of playing field, and so forth), the structure of the game (i.e. number of players, groupings of players, strategies, and so forth), and the game's personal requirements (i.e. what the game requires of the player — motor skills, fitness levels, numeracy, social skills, and so forth). Games for understanding This is a classification scheme proposed by Werner and Alomond that classifies games according to their strategies. It divides games into target games (e.g. archery); net or wall games (e.g. tennis); striking and field games (e.g. cricket); and invasion games (e.g. football). Core content This is a classification scheme proposed by Allison and Barrett that categorizes games by their form (i.e. whether they are novel games proposed by the teacher or children, or whether they are existing games already widely played), by the movement skills that they require, by the "movement concepts" and game tactics that they require, and by the educational results of the game. Developmental games This is a classification scheme proposed by Gallahue and Celand that classifies games into four developmental levels, as part of an overall educational strategy of applying, reinforcing, and implementing movement and sports skills. The levels, in ascending order, are "low-level", "complex", "lead-up", and "official sports". Video games There are several methods of classifying video games, alongside the system of video game genres commonly used by retailers and player communities. Solomon puts forward a "commonsense, but broad" classification of video games, into simulations (the game reflects reality), abstract games (the game itself is the focus of interest), and sports. In addition to these, he points out that games (in general, not just video games) fall into classes according to the number of players. Games with two players encompass board games such as chess. Games with multiple players encompass card games such as poker, and marketed family games such as Monopoly and Scrabble. Puzzles and Solitaire are one-player games. He also includes zero-player games, such as Conway's Game of Life, although acknowledging that others argue