Heating With Hot Water Containers

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 kinda, topic came up 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