Philips AVENT Natural variable flow nipple has a slot shaped opening that can change the flow of milk. Simply turn the bottle to line up the i, ii, or iii marking on the nipple with your baby's nose to choose between slow, medium, and fast flow. The wide, breast shaped nipples promote natural latch on so you can easily combine breastfeeding and bottle feeding., and the unique comfort petals inside the nipple increase softness and flexibility without nipple collapse. The nipples are made of flavorless, odorless silicone that resists discoloring and stickiness.