"A hydration pack is a type of hydration system built as a backpack or waist pack containing a reservoir or \"bladder\" commonly made of rubber or flexible plastic. The reservoir contains a capped mouth for filling with liquid and a hose that allows the wearer to drink hands-free. Most hoses end with a \"bite valve\" that opens when the user bites down on it; the valve may be protected by a dust cover. Some hydration packs are insulated to keep water from freezing or becoming warm."@en . . .