The Nûby™ Non-Drip™ nipple is uniquely designed to prevent drips and spills. With a pressure-sensitive valve feature built right into the nipple, fluid only flows out of nipple with sucking actions of baby. This special valve closes when sucking action ceases, ensuring a leak-resistant bottle. The Nûby™ nipple massages infant's gums, aiding the teething process. With your baby's comfort in mind, an anti-colic feature was designed into the nipple, helping to prevent ingestion of air by baby, which may cause colic.