Dr. Brown’s Standard bottle is using an 2-piece internal Vent mechanism. This facilitates positive pressure flow feeding, which is similar to breastfeeding. During feeds, air is transferred from the teat collar through the Vent system to the back of the bottle. The air circumvents the breast milk or formula and minimizes oxidation. Also, the essential nutrients of the feed like vitamins a, c and e are preserved. As babies don’t have to fight the discomfort of ingesting air BUBBLES, feeds are longer and easier. This bottle helps significantly reduce gas, colic and spit-ups. Why you’ll love it babies feed comfortably as the Vent system helps prevent discomfort caused air BUBBLES. Features Made of polypropylene unique 2-piece internal Vent system that keeps air at bayretains essential nutrients in fluidseffortless feeding without discomfort of ingesting air bubbleshelps reduce gas, colic and spit-ups bpa-proof.