Given the different contractions of the two different metals as a result of the cold, when it gets cold enough iron cannonballs will fall off a brass monkey.

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The sessions I attended, in a room called Brass Monkey (the theme was New York bars), had the aspect of a nerdy summer camp.

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Which makes no sense at all as a monkey is, in archaic naval parlance, a brass triangle which iron cannon balls are piled up inside of.

FORBES: Why Wind Power Doesn't Work