Various facilities can be found throughout the Pokémon world such as Pokémon Centers, Poké Marts, and Gyms. At a Pokémon Center, the player can have their Pokémon healed for free and access the PC, where players can organize their collection of Pokémon, store and withdraw items, and have their Pokédex rated. Prior to X and Y, this is also where players can link up with other game cartridges or cards to battle or trade with other players. Poké Marts are shops where players can buy items with the money they win during battles; certain cities may have specialized shops, like a pharmacy or a department store. Periodically, a town will contain a Pokémon Gym, which houses a powerful Trainer known as a Gym Leader. Victory against him/her grants the player a Gym Badge and typically advances the plot. After collecting eight Gym Badges, the player may challenge the region's Elite Four and Champion; defeating the Champion finishes the main story. Gyms are absent in Sun and Moon, being replaced by trials which serve a similar purpose. As the name implies, instead of a straightforward battle, the player undergoes a trial. Upon completion the player receives a Z-crystal, a crystal which grants a Pokémon access to an extremely powerful move.