Designed to function like a real cow's teat, moving all the time while the calf is sucking. Micro-thin soft but durable. Teats feel natural and comfortable to the calf, almost identical to mother's. Has an opening on each side so nipple does not collapse and is leak resistant. The unique flap holds milk in the nipple. Will work with tube or gravity feeding. Especially helpful for very young calves. Nipples are screw on.