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As climate change causes temperatures to climb, I've found it more and more important to make sure I have not just water, but ice water in high quality thermoses when spending time outdoors. While this means I can't store as much water per area of volume since ice expands, cold water is more refreshing on a hot summer day, and importantly, actively and quickly helps to cool your core body temperature. In a high quality thermos stuffed with as much ice as you can put in and still allow it to close with the remaining space filled with water, the ice can last for more than 24 hours. Note this won't happen if you only use three or four cubes of ice--the remaining water will quickly melt that, even without any outside heat getting into the thermos (though it will still make the water cooler). A good quality thermos will use vacuum sealing to help insulate its contents.