No. Exposing the canteen to boiling temperatures could cause it to crack or split. However, there is a lightweight Tritan polymer canteen that can tolerate boiling temperatures.
the metal.
they have canvas sides to put a bit of water on for evaporative cooling of water inside.
It is commonplace to put salt in water when boiling most foods. The reason why you put salt in with boiling water and vegetables is to help them retain moisture.
They would get water by camping out near streams and rivers. Then, they would put the water in their handmade gourd canteens. That's how they got water.
You put them in the water after the it is boiling not before
It depends on the plastic. Some plastics might melt. Or burn. Or release toxic chemicals into the food. Others won't. Teflon's pretty darn safe, for example, though you probably still don't want to get it so hot it melts (Teflon does not burn).
noddles
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Probably what they wanted to say is something like "Do not put your hand into the boiling water."
No, boiling point is not affected by volume. What will change is the amount of energy you need to put in to get it to boiling point.
it makes a thundering noise because you are mixing freezing water with boiling hot water
It melts