Dr. Browns Glass Bottle 2 pack features a patented two-piece internal vent system that works unlike any other bottle design. The vent system fully vents the bottle for vacuum-free feeding, which is called positive-pressure flow, similar to breastfeeding. As the baby feeds, air is channeled from the nipple collar through the vent system, bypassing the breastmilk or formula, to the back of the bottle. Air never mixes with the breastmilk or formula which minimizes oxidation and helps maintain essential nutrients like vitamins C, A and E. Babies feed more comfortably because the vent system allows them to nurse without fighting the negative effects of a vacuum or the discomfort of ingesting air bubbles. This helps to reduce feeding problems like colic, spit-up, burping, and gas.