The player can use his or her Pokémon to battle other Pokémon. When the player encounters a wild Pokémon or is challenged by a trainer, the screen switches to a turn-based battle screen that displays the engaged Pokémon. During a battle, the player may select a maneuver for his or her Pokémon to fight using one of four moves, use an item, switch his or her active Pokémon, or attempt to flee. Pokémon have hit points (HP); when a Pokémon's HP is reduced to zero, it faints and can no longer battle until it is revived. Once an enemy Pokémon faints, the player's Pokémon involved in the battle receive a certain number of experience points (EXP). After accumulating enough EXP, a Pokémon will level up. A Pokémon's level controls its physical properties, such as the battle statistics acquired, and the moves it has learned. At certain levels, the Pokémon may also evolve. These evolutions affect the statistics and the levels at which new moves are learned (higher levels of evolution gain more statistics per level, although they may not learn new moves as early, if at all, compared with the lower levels of evolution).