Boiling to death is a method of capital punishment.
The condemned would be placed in a giant cauldron of cold water, tied up to prevent escape. The executioner then lit a fire under the cauldron as to allow the water to slowly heat up. It was a very painful and slow method of execution.
While not as widespread or frequent as other methods, this mode of capital punishment has seen widespread use in both Europe and Asia over the past two or three thousand years. For example, it was a legal form of capital punishment during the reign of Henry VIII, reserved for poisoners.
The image of cannibals cooking people alive to eat them is most likely a myth. Historically, cannibalism has been practiced by cultures that lack the engineering wherewithal or inclination to cast an iron port large enough to be used for this purpose.
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