Take an icecube tray and fill it up. Take 3 inch(length of thread my vary depending on nipple length and gage) pieces of thread or non- flavored, non-waxed floss and lay them across each individual ice cube, pushing them in about half way. Put tray in freezer until frozen. When cubes are completely frozen remove tray and run under luke warm water for 2 seconds releasing cubes from tray. Grab your nipple hardening cubes by ends of thread and tie around nipple insuring a snug but firm fit. Adjustments may be needed as nipple hardening occurs. Repeat as necessary. Enjoy your hard nipples.