GLASS ANTI-COLIC BABY BOTTLE. Anti-Colic internal vent system is clinically proven to reduce colic. Decreases spit-up, burping and gas DURABLE BOROSILICATE GLASS. Dr. Brown's Glass bottles are heat and thermal shock-resistant, which can withstand hot and cold temperatures, along with gradual temperature changes CONSISTENT FLOW RATE. Vacuum-free feeding is closest to breastfeeding and Dr. Brown's silicone nipples provide a consistent flow so babies can feed at their own pace. PRESERVES NUTRIENTS. The anti-colic vent system is proven to help preserve bottle milk nutrients and aids in digestion for a better night's sleep BREASTFEEDING BOTTLE. The breast-like nipple shape encourages a proper latch for a more natural bottle-feeding experience and eases the transition from breast to bottle and back again