Comment by closewith

> Even if you don't have a metal pot you can do stuff like heating rocks and then stacking them on cool rocks inside a plastic, glass, ceramic, wood, etc container filled with water to get to a boil.

That can get you sterile water, although it's extremely difficult to do and involves many more rocks than you'd imagine easily 5x the mass of rock to water to get a rolling boil for a full minute, but it doesn't get you clean water. Now you have sterile water with a lot of potentially very unpleasant dissolved solids. Certainly not something you'll be using to feed an infant.