Unsettled and irritable behavior, known as fussing, is by far the most commonly known crying behavior in babies. Clinical studies show that thePHILIPS AVENTbottle significantly reduces fussing and colic. Polypropylene (PP) is a more plyablematerial that has a milky hue color. Philips AVENT bottles made of this material come with an additional adaptor ring to ensure the unique skirt on the teat always fits the ring of the bottle, guaranteeing the anti-colic feature.As your baby feeds, the unique skirt on the AVENT Teat flexes to allow air into the bottle instead of your baby's tummy.Unlike other bottles, the clinically proven anti-colic system is now integrated into the nipple, making it easier to assemble the bottle correctly. As your baby feeds, the unique valve on the nipple opens to allow air into the bottle instead of your baby's tummy.