Water chip

The Water Chip is a device which controls the water recycling and purification process in all Vaults. The original Fallout game began when Vault 13's water chip apparently broke and there were no replacements. The original Vault Dweller is sent to find a new one, and finds one in Vault 12, the vault underneath Necropolis and returns it to Vault 13. Due to a mix up in shipping, Vault City attained Vault 13's additional Water chips, in exchange for their additional Garden of Eden Creation Kit.

Through another strange twist in fate, primarily due to the Guardian of forever, the Chosen One actually broke the original Vault 13 water chip, setting the story in motion. (Whether or not this is canon is debatable.)