Years ago, when the ships had sails and cannons, they would try to stack the cannon balls near the cannons.  With nothing to hold them, they would roll around.  So someone made a plate, made out of iron with indentations to hold the cannon balls in place on the bottom layer.  They could then stack the cannon balls up in a prymid.  This device was called a monkey (I have NO idea why).  One catch....with the salt in the air, the cannon balls would rust to the monkey which was no good.  So the inventor tried a different metal...BRASS.  That work well.  The cannon balls no longer froze to that brass plate.  WELL...there is a difference in expansion for brass and iron.  The brass would get cold first and it would contract.  When this happened, the cannon balls woud pop off of the brass monkey....Therefore the saying was born "it is cold enough to freeze the balls off of a brass monkey".

Good one, David....