Natural Response nipple: The unique Philips Avent bottle nipple opening only releases milk when the baby actively drinks. Works like the breast: The teat releases milk when baby actively drinks, this is different from the traditional free-flow teats. As with breastfeeding, it can take time for a baby to get used to this. Anti-colic valve: The baby bottle is designed to keep air away from the baby's tummy during feeding No-drip nipple: The bottle-feeding nipple only opens to release milk when the baby is feeding. Heat resistant glass: Made of borosilicate glass, the Philips Avent glass bottle is heat and thermal shock resistant.