We designed the bottle nipple to act like a mother's breast, to make it easier to combine breastfeeding and bottle feeding and support your baby's unique, instinctive sucking rhythm. Milk is only released when your baby actively sucks. When your baby stops to swallow and breathe, the milk stops flowing too. The additional AirFree vent makes it easier to feed upright by filling the nipple with milk, not air, which reduces the amount of air swallowed and reduces the risk of colic, gas and reflux