It was necessary to keep a good supply of canon balls near the cannon on
war ships. But how to prevent them from rolling about the deck was the
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 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/rolling from under the others. The solution was a metal plate
with 16 round indentations, called a Monkey.

But if this plate was 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 cannon
balls would come right off the monkey.

Thus, it was quite literally,cold enough to freeze the balls off a brass
monkey. And all this time,
you thought that was a vulgar expression, didn't you? You must send this
fabulous bit of historical knowledge to at least ten unsuspecting
friends.