One of the wonderful properties of water is that it holds a steady temperature at 212 degrees Fahrenheit which is lower than the combustion point of it’s container. This amazing property has the effect of protecting the vessel that contains it so fire will not burn through the barrier that holds it. In modern terms, it acts like a heat-sink that draws heat away.
While being heated, containers like a cabbage leaf or a paper cup may burn down to the level of the water so it becomes a one-use container. A tougher container like a bamboo node or coconut shell can be used to boil water as well and may be used more than once.
There are some guidelines that should be observed: the vessel should not leak fluids that would extinguish the flame or coal: if the vessel is flexible or fragile, it will need to be supported in the process. The best way to support the cabbage leaf is to place it on live coals that you have prepared in a cup-shaped mound. Immediately fill it with water to protect the leaf and and hold it in place. Once the water comes to a boil, the challenge will be to extract it from the vessel. In this case a smaller vessel like a spoon or small cup can be used to draw out the hot water for use.
An alternate method of boiling water would be to put dry rocks into a fire and heat them until very hot, then using a pair of tongs, transfer them into the water to heat it. It may take several rocks, but eventually it can be made to boil.
So, whether to heat from the outside, or from the inside, you now know how to boil water in a cabbage leaf. Have fun!