Brown's Natural Flow Standard Glass Bottles offer a wonderful feeding experience with innovative vent technology. Bottles use a patented two-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 BPA-Free Baby Bottle 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 on these Dr. Brown's Natural Flow Standard Glass Bottles eliminates air bubbles in the formula or milk, which helps to reduce feeding problems like colic, spit-up, burping and gas.InstructionsWash with warm soapy water before using.