Fixing a stained granite kitchen counter depends on the type of stain. For oil-based stains (e.g., cooking oil, grease), create a paste of baking soda and water, apply it to the stain, cover with plastic wrap, and let it sit overnight. Rinse and repeat if necessary. For water-based stains (e.g., coffee, wine), use a mixture of hydrogen peroxide and a few drops of ammonia. Apply it to the stain, let it sit for 5-10 minutes, then rinse and repeat. For stubborn stains, consider a commercial granite cleaner or consult a professional. To prevent future stains, seal your granite regularly and blot spills promptly.