Suppose you don't have a pot or any other fire proof container to boil water in, but you need to boil it for safe drinking water. How else can you boil the water?
A watertight container is what is required here. The best thing is metal but if we're without this you can use
- plastic bottles on the shoreline
- folded paper
- a hollowed out and charred piece of wood*
- broad leaves shaped using twigs*
- birch bark*
- dig up some clay, create a bowl and throw it in the fire
- carve a wooden bowl
- use strips of wood, bound together with cordage and plugged with clay*
In order to heat up your water, if your container is fireproof (metal, clay) throw it in the flames and away you go.
If you're worried about your container, an efficient way of heating water is to heat up rocks in the fire and then put them in the water.
*One note on heating rocks, if they contain sand or water, they may explode from heating, so be careful with your choices!
edit Don't have a link but was on a survival course and the instructor advised that seaweed* can be used as well to boil water it
*edit 2 I'm sure it goes without saying but when hollowing out wood, using leaves or seaweed to boil water it is essential that the materials used are non-toxic! Do not use leaves/wood in your cooking from plants that you are not 100% positive on the ID
You can actually boil water in any material that has a higher ignition point than 220 degree F (100 degree C) because the water will hold the temperature at its maximum boiling temperature.