"These Nipple Suction Cups are small but nice. Easy too: Lick them, squeeze them, put them on your nipples and they will suck them in. Since the wand of these suckers is relatively thick, the suction these little cups provide is actually quite good. Internal diameter 10 mm so relatively small. Intended mainly to enlarge / sensitize the tip of the nipple only. A larger diameter would lessen the suction.