SPECIAL NIPPLES WITH ANTI-COLIC AIR EXCHANGE SYSTEM can reduce the intake of air to baby's stomach with advanced system. The shape mimics the mother's breast during suction. As your baby feeds, the unique valve on the optimal teat flexes to allow air into the bottle instead of your babys tummy. These BPA-free nipples are made of transparent food grade silicone. Heat resistant, non-toxic and odourless.