how do butane hand warmers work?

Question

i mean after you light them you put them in a cloth bag and the fire dont go out nor dose the bag catch on fire

Answer 1

The fire is not big enough to catch fire, but it will go out after a while. But according to this guy you can get many hours of good heating. (scroll down to "heaters")
http://www.rocketroberts.com/astro/warm.htm
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Answer 2

Butane Hand Warmer