The Deuter Streamer Thermo Bag is a padded, zippered pouch lined with Mylar, of which the material reflective blankets are made. It's a little sleeping bag for the bladder which will keep hot and cool liquids in their preferred states longer. How much longer depends on the original temperature of the liquid and the temperature of the day. Will fit both 2 and 3 liter Deuter/Source hydration bladders.