These unique nipples have a hollow cone-shape that provides a choice of feeding rates. The valve allows air to be freely admitted, but prevents milk from being squirted out the air vent. Nipple cannot be pulled from bottle. No mess, no waste. Veri-speed nipple can be cut with scissors to adjust feeding rate. For a fast flow, cut off complete tip so weaker lambs can drink. For a slower flow, snip a little off the tip. If no hole appears (as can occasionally happen), pierce with hot needle heated over a candle or match flame. Easily screws on most size bottles including the 2 liter plastic pop bottle. Made in New Zealand.