The Dr. Browns Natural Flow baby bottle is the only baby bottle that features a patented internal vent system. This vent eliminates the air bubbles and vacuum effect to allow positive-pressure feeding with no air - similar to breast-feeding. Designed to Reduce colic spit-up burping and gas Prevent air bubbles from forming in milk or formula Help reduce fluid in the ear Prevent nipple collapse Features Made from BPA free polypropylene plastic Easy to clean and top-rack dishwasher safe Includes a high-grade Level-One silicone nipple patented two-piece internal vent travel disk cap cleaning brush and instructions