Nuk New Classic Bottle 6-18m 250ml is the traditional slim classic bottle but now comes with a wide neck for use with a Nuk first choice teat. Nuk air system is an air vent which compensates for the difference in air pressure in the bottle. This not only helps to prevent the teat collapsing, but also the harmful swallowing of air. Different hole sizes ensure a consistent flow of feed even with different consistencies of feeds. The bottle cap ensures hygiene.