I have two thermoses. One is a proper vacuum thermos, and in freezing weather its outside turns into a piece of icy metal that gives off almost no heat. The other has a plastic cup-shaped top that screws on and keeps the water inside. The plastic gets warm and gives off heat.

I discovered that this works really well when you get cold on a hike. A regular bottle filled with boiling water has a large surface area, so it loses heat quickly. With my thermos, most of the body is vacuum-insulated, and only the small plastic top gives off heat. So the heat lasts longer.

If I hold the warm plastic part between my thighs inside my sleeping bag, it keeps me warm.

I guess this is what you call a life hack!

Recently, I got cold during the night, and this little heater kept me warm enough to sleep for six hours.