Our suction cups are made of black soft rubber, they can be squeezed to push the air out and then placed over the nipple. A nice suction draws and stretches the nipple up into the rubber tube bringing the blood into the nipples for a tingling sensation This also make the nipples much more sensitive after you take these off.
Apply petroleum jelly, lip balm or thick lubricant around the nipple before placing suction cup for maximum grip.
S - 5/16" opening (round top)
M - 3/8" opening (round top)
L - 5/8" opening (flat Top)
XL - 3/4" opening (flat Top)