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It's peculiar coloration is the result of its iron rich body rusting. The reason the rust is only one one side is because of its body also being divided magnetically, the Iron thus comprising one hemisphere and all other elements the other. Were it not weighed down by this heavy iron ore it is likely that Voltorb and Electrode would have the Levitate ability. When it evolves its magnetic poles switch, and in the process it loses its rust and, for a time, is pure white until the opposite hemisphere, where the iron atoms have migrated, rust once again.
Its resemblance to a Pokeball is a coincidence.
