When you start using it, black stains may appear if you rub it. This phenomenon occurs because the oxide film, a characteristic of stainless steel, is not yet stabilized. After using it for about 1-2 weeks, it will become less noticeable. However, to resolve it as quickly as possible, add citric acid (or vinegar) to the inner container in a 10:1 ratio (10 parts boiling water to 1 part citric acid) and boil for a few minutes. After leaving it for about an hour, wash it with dishwashing detergent for effective results.