The flutter valve nipple, introduced by sheep ranchers in New Zealand, is the best nipple we've found for feeding weak or newborn kids. The soft red rubber teat is perfect for the kid to suck. Plastic base screws onto 28 mm glass or plastic bottles (disposable pop bottles, etc. -- a very common size). A metal bearing "valve" inside the nipple lets air flow in smoothly but controls milk flow. Since the nipple won't drip, kids are less likely to choke. Excellent for kid-feeding for the first five days or for weak kids. Keep several on hand for starting newborns. Also available in bulk.