Brass Monkey

In the heyday of sailing ships, all war ships and many
freighters carried iron cannons. Those cannons fired round
iron cannon balls. It was necessary to keep a good supply
near the cannon. However, how to prevent them from rolling
about the deck? The best storage method devised was a
square-based pyramid with one ball on top, resting on four
resting on nine, which rested on sixteen. Thus, a supply
of 30 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 or rolling from under
the others. The solution was a metal plate called a
'Monkey' with 16 round indentations. However, if this
plate were made of iron, the iron balls would quickly rust
to it. The solution to the rusting problem was to make
'Brass Monkeys.' Few landlubbers realize that brass
contracts much more and much faster than iron when
chilled. Consequently, when the temperature dropped too far,
the brass indentations would shrink so much that the iron cannonballs
would come right off the monkey; thus, it was quite
literally, 'Cold enough to freeze the balls off a brass
monkey.' (All this time, you thought that was an improper
expression, didn't you.)