While the milk is cooling you need to 'sterilize' any containers or utensils that will touch the milk/yogurt. The addition of unwanted bacteria can inhibit the coagulation of the yogurt, so this is an important step. Simply boil some water (we used a pressure cooker) and rinse all items that you will be using. We simply placed all the items in a sink and slowly poured the hot water over all the surfaces.