Allows baby to control the speed of milk drinking, just like when breastfeeding. The unique opening and tip of the nipple releases milk only when baby is actively drinking. No milk comes out when swallowing and breathing is suspended. Baby can drink, swallow and breathe at a natural rhythm, just like when breastfeeding. The ergonomic shape makes it comfortable for parents to hold and easy for little fingers to grip. Assembly is simple, requiring only a few parts, and the wide neck makes for easy filling and cleaning. Made from polypropylene and BPA-free, the Nature Series bottles can be used with all the teats and caps in the Nature Series. Natural Response nipples have a flow rate of 1-5. Ultra air nipples have 4 large air holes in the shroud to allow baby's skin to breathe and maximize air flow.