Best anti leak suction facilitating mechanism weight straw; allows smooth flow of milk only when sucked; prevents air to get in the mouth of the baby- no air swallowing or feeding problems; made from food grade silicone - safe, durable and elastic. Strategically designed adaptor/valve with double air vent- prevents air to enter or trap in the bottle; holds the straw in the bottle by connecting it to the nipple (nipple not provided).