The Dr. Brown"s Natural Flow Standard Baby Bottles offer a wonderful feeding experience with innovative vent technology. Dr Brown"s bottles use a patented 2-piece internal vent system for your baby"s comfort. This system creates positive-pressure flow for vacuum-free feeding that is similar to breastfeeding. The breast milk or formula flows freely without nipple collapse. As the baby feeds, air is channeled from the nipple collar through the vent system to the back of the bottle. Air never mixes with the breast milk or formula so oxidation is prevented, thereby maintaining essential vitamins such as C, A and E. The vent system eliminates air bubbles in the formula or milk, which helps to reduce feeding problems like colic, spit-up, burping and gas. Dr. Brown"s bottles are completely BPA, PVC, lead and phthalate free for your baby"s safety. Each bottle comes with 1 Level-One silicone nipple, 1 two-piece patented internal vent system. 6 pk includes 1 cleaning brush and instructions.