Finally, how you boil water in a bottle is important. If you warp the bottle with too much heat, the lid won't screw back on properly and the bottle loses its water transporting ability. The trick when boiling in these kinds of containers is to place the vessel beside the fire, not in it. Just snug the bottle up in the ashes on the side of the fire, wait for the big bubbles to start jumping, and then feed the fire so that the boil continues for 10 minutes. This will kill all the critters in the water that could sicken you, and keep the bottle cool enough to prevent warping.