Brass Monkeys:
It was necessary to keep a good supply of cannon balls near the
cannon on old war ships. But how to prevent them from rolling about
the deck was a real problem. The best storage method
devised was to
stack them as a square-based pyramid; with one ball on top, resting
on four, resting on nine, which rested on sixteen. Thus, a supply of
thirty cannon balls could be stacked in a small area right next to
the cannon.
There was only one problem - how to prevent the bottom layer from sliding/rolling from under the others. The solution was a metal plate with sixteen round indentations, called a Monkey. But if this plate was made out of iron, the iron balls would quickly rust to it. The solution to the rusting problem was to make the Monkeys out of brass.
Few landlubbers realize that brass contracts much more and much faster than iron when it is chilled. Consequently, when the temperature dropped too far, the brass indentations would shrink so much that the iron cannon balls would fall right off the Monkey. Thus it was quite literally, cold enough to freeze the balls off a brass Monkey!
(Thanks to CW4 (Ret) Roy Myers of the Oregon Military Museum.)