Peach teat calf nipples are designed to act like a real cow's teat, moving all the time while the calf is suckling, never closing in the relaxed state. The teat allows more milk to pass through then a real cow's teat, allowing the calf to suckle more intensely & simulate the flow of saliva to improve ph-levels in the stomach for better digestion. Since the peach teat's unique, patented internal valve holds milk in the teat, it makes it more responsive to the calf's needs. The Black pull thru teat has a specially made angle cut on the back of the teat, so it fits into narrow fitting feeders where there is limited room for a teat (I.E. Milk bar feeders). the teat is pulled through the hole, & the cut angle allows milk to flow freely into the back of the teat.