The Medium Flow Nûby™ Silicone Nipple features a medium valve opening that is prefect for milk. It's made of soft silicone and features a natural shape which feels more like a mother's breast. This natural shape and feel promotes healthy, active feeding, and makes the transition between breastfeeding and bottle feeding easier. In addition, the Medium Flow Nûby™ Silicone Nipple features anti-colic vents along the base which help prevent a vacuum from forming in the bottle. This reduces the amount of air the baby ingests, and thus, reduces gas and the chances of colic.