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In Generation 1, it was possible to evolve certain evolutionary stone-requirement Pokemon without using an evolutionary stone.
By leveling up the Pokemon in question during battle, and then finishing the battle with another specific Pokemon, the game would confuse the index number of the Pokemon with the index number of the item, and trigger the evolution. The list of Pokemon corresponding to particular evolution stones is as follows.
>Moon Stone: Exeggutor
>Thunder Stone: Growlithe
>Water Stone: Onix
>Leaf Stone: Psyduck
