During winter camping and hiking, a bottle full of hot water is better then heated rocks. We usually will put the hot bottle into a couple of spare socks then into the sleeping bag. These stay warm for many hours and in the morning, you have water that is not frozen and is ready for use.
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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