The round nipple promotes a similar tongue placement and sucking technique when breastfeeding, since the round shape allows the sides of the tongue to elevate and cup around the nipple, just like it does during breastfeeding.
The nipple is equipped with a valve, letting out air when the baby closes down on the nipple. This ventilation system causes the air from inside the nipple to be pushed out through the valve thereby flattening the nipple to shape naturally after the baby's oral cavity.
The valve is also the reason why water can be presented inside the nipple after cleaning and sterilization. If this is the case, simply just squeeze the nipple flat to press out the excess water.
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