A unique patented skirt system, with a one-way air valve at its base, flexes to allow air into the bottle, replacing the milk at your baby's own suckling rhythm and eliminating the vacuum. Your baby can feed continuously as at the breast, without having to 'come up for air'. The whistling sound you hear is your guarantee that air is going into the bottle and not into your baby's tummy. It's the air babies swallow during feeding that can cause the discomfort, crying and fussing associated with colic.