You stored tomato sauce in a plastic container and now you regret it. The leftovers have left their mark. Fill the container with warm water. Add 2-4 tablespoons of baking soda, depending on the container size, and leave to soak. This should get rid of the worst stains. You may need to use a little elbow grease if the marks are bad. Sprinkle a little baking soda onto a clean sponge and scrub.