On the Fly Bottle Warmer - for small 4 oz. bottles.  On The Fly™ bottle warmer was designed for mothers and fathers ‘on the go’ that desire a functional and chic product for their baby. Whether parents are running errands, traveling, working, or playing this product offers a simple solution and better alternative for heating baby bottles. The bottle sock is a two-layered stretch fabric product, which is used with the newer, wider bottles such as ADVENT™, GentleFlow™ from Gerber™ and BornFree™. The warmer is inserted between the two layers of fabric and is intended to slowly heat and maintain a tepid temperature for water with mixed formula or premixed formula. The warmers can be used many times on a flight or on an outing because they maintain their warmth for up to eight hours. Our product provides a hassle-free and electricity-free way to heat your baby bottles. On The Fly™ is packaged in recyclable materials so that we can do our part to help the environment. 

Comes with 1 sock and 5 inserts.  Perfect for travel - NO electricity needed.  We tried these out and while it takes about 20-30 minutes to warm, it stays warm for a long time.  Just what is needed for traveling with baby.