Dr. Brown's Silicone Narrow Neck Options Nipples are made of super-soft, high-grade silicone, each nipple level provides a different flow rate. From the Preemie Flow nipple, which limits the flow to tiny mouths to the Y-Cut nipple which literally utilizes a “Y”-shaped opening to allow for greater milk flow, you’ll find the bottle nipple that’s the perfect fit for your baby’s feeding needs. Compatible exclusively with Dr. Brown's Narrow neck options bottles. Dr brownComes in 4 different sizes.