Not exactly do or die, but last weekend I had to use the bandana out of my PSK to pre-filter spring runoff water from a mountain fed creek that had a lot of sediment etc which could of proven very disruptive to the digestive system if I had just treated the water with purification tablets alone.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock