You can boil water in the wide mouthed nalgene bottles. Find a dry rock that will easily fit into the bottle and throw it into the fire. Once it is scorching hot take a green stick. Bend the stick in half and pickup the rock in the stick. Tie the stick tightly to the rock with cordage and dip the rock into the nalgene bottle. Hold until it boils.

Another method is to put a few layers of bark and sand/dirt in the bottom of the nalgene and use a bark trough to guide the rock to the bottom WITHOUT touching the sides. The water will boil in short order.

If the rock accidentally touches the plastic walls DUMP the contents or it will likely melt through the plastic. I have yet to do this but I suspect it is only a matter of time.

-NIM