
BornFree® leak proof BPA-Free baby bottles come in plastic or glass and feature an innovative venting system designed to promote ActiveFlow™ and and help minimize the painful symptoms of colic.

The BornFree® simple two part venting system is specially designed to reduce the vacuum build-up inside bottles that often can lead to gas, spit-up and painful colic in newborns. The vent allows a continuous free flow of air into the teat while the baby is feeding.
BROWN & MAGNUSSON "On the physics of the infant feeding bottle and middle ear sequela" 2000
In bottles without a vent, as your baby feeds, a vacuum is created. This negative pressure makes it hard to feed comfortably. Your baby has to overcome the vacuum by sucking unnaturally hard to create a flow of liquid.
This is one of the main reasons why babies develop post-feeding discomfort usually associated with colic, particularly if they are smaller infants or have an immature sucking reflex. It makes them tired and irritable and thus they often cry.