==Exchanging key items==
The exchanging of one key item for another has been present since [[Generation I]]: a [[Bike Voucher]], a seemingly useless item, was given to the player to exchange for a [[Bike]]. Key item exchanging can only happen once, with the exception of Pokémon [[Pokémon Ruby and Sapphire|Ruby, Sapphire]] and [[Pokémon Emerald|Emerald]], in which the player
could swap the two [[bike]]s (the [[Mach Bike]] and the [[Acro Bike]]) as they choosed. In any case, only one or two items could be exchanged.