> A hot water bottle (or hottie[1]) is a container filled with hot water and sealed with a stopper, used to provide warmth, typically whilst in bed, but also for the application of heat to a specific part of the body.
(repost of sorts, this came up again in conversation)
I thought I've read people suggest doing something like this to stay warm while camping
found idea at / credit to Low Tech Magazine but their article has immodest pics so wasn't gonna link it