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 occassionally 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.