The next time someone tell's you it's cold enough to freeze the balls off a brass monkey, you point them here. This is a brass monkey. Notice how it's holding the balls. If it were to be cold enough, the brass monkey would contract, and the cannon balls would fall off and go rolling around.
I think this monkey is actually made of iron... but you get the idea.
All of this is supposedly bunk of course... but the only other image I can think of when I hear that phrase is just too totally unreasonable to consider.