NATHANA(R) INSULATED SOFT FLASK WITH BITE TOP 18 ounce :: Thirsty for maximum hydration? Your handy Nathan Insulated Soft Flask with Bite Top 18 ounce has you covered. Insulated to keep fluids cooler for 38% longer than non-insulated versions an easy bite valve keeps liquids inside until you need a drink. Fits perfectly with all Nathan hydration vests.