CANNONBALLS ON SAILING SHIPS had a tendency to roll around the deck. Stacking them in a pyramid helped, not only to keep them in a neat pile but also because the friction between them helped stop them rolling away.
To make sure they didn’t roll around the deck, the powder monkeys – the sailors charged with maintaining and loading gunpowder into the ship’s cannon – would place a triangular retainer, also known as a monkey, around the base of the pyramid.
Sometimes these retaining monkeys were made of wood, but they would rot. If made of iron, they would rust. The material of choice was brass, but brass had one unfortunate characteristic.
In very cold weather, the brass would contract. That’s when you knew it was cold enough to freeze the balls off a brass monkey.
