Dr. Brown"s bottle features a patented two-piece internal vent system that works unlike any other bottle design. The vent system fully vents our bottle for vacuum free feeding, which we call positive-pressure flow, similar to breastfeeding. As the baby feeds, air is channeled from the nipple collar through the vent system, bypassing the breast milk or formula, to the back of the bottle. Air never mixes with the breast milk 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. The breast milk or formula flows freely, without nipple collapse. Includes two 4 ounce glass bottles with two level one nipples, two 2 piece internal vents and one nipple cleaning brush. BPA Free.