A hyration pack is a type of hydration system built as a backpack or waistpack 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. Reservoir sizes commonly range from 1.5 to 3.0 litres.