They’re these little bags full of some kind of coarse powder that heats up when it’s exposed to air. There’s no metal piece or anything you have to light. You just take them out of the plastic package and shake them a bit to get the air flowing and they heat up. HotHands is the brand name.

I just got back from camping for a single night at 15°F (-9°C). In the middle of the night, I got the bright idea to move my CPAP and hose into my sleeping bag with me I figured the warm air inside the bag would prevent frostbite on the end of my nose. It worked perfectly! Until I farted. I consider...