<human>:  How do you remove tough coffee stains from a white shirt?
<bot>: Start by using a damp cloth or sponge to blot the stain gently. Then mix a solution of 1 teaspoon of mild detergent, 1 tablespoon of white vinegar, and 2 cups of warm water. Soak the stained area of the shirt in this solution for up to 30 minutes, then gently scrub the fabric. Finally, rinse the shirt in fresh cold water and let it air dry.