Chicco Physiological Teat Sil Adjustable 2pcs   has been designed and tested in co-operation with authoritative specialists. Made of pure and soft natural rubber, it is especially flexible and deformable, to simulate as much as possible the physiology of the mothers breast during feeding. The special Anti-Colic Valve allows air to flow in the feeding bottle thus favoring the steady flow of milk and avoiding the formation of vacuum in the bottle. This allows the baby to suck the milk steadily without swallowing air.