Each package includes four natural silicone nipples. The Slow Flow Stage 1 nipples are for 0+ months. The Medium Flow Stage 2 nipples are for 3+ months. The Fast Flow Stage 3 nipples are for 8+ months. As your baby grows, you can replace the Stage 1 nipples with faster flow ones. Micro Air Vents allow air to return to the bottle, preventing nipple collapse and keeping air out of baby's tummy. Sensitive Response nipples are soft and flexible; and they adjust to baby's pressure while suckling for the proper flow of liquid. Dishwasher safe (top rack only). Wash and sterilize all parts thoroughly after each use. Nipples are made in Mexico. When bottle-feeding your baby, make sure the nipple hole is of the right size. If your child appears to be sucking too hard, you probably need a fast/medium-flow nipple. An overly resistant nipple could collapse under your baby's strong sucking motion. On the other hand, if your baby seems to be sputtering and gulping often, your baby could need a slow-flow nipple. For a newborn, you know the nipple size is right when the milk comes out in a spray for a second or two when the bottle is inverted, and then trickles down to drops. Do not attempt to enlarge the hole in the nipple; this may cause tearing and present a choking hazard.