1. If there is tea or milk in the cup, you can wash it off. Put some hot water in it and soak it in baking soda 10 minutes or so. Soak baking soda and hot water for a while, and brush it inside with a brush.

2. You can also pour a little detergent on the scouring pad or wash it clean. If you have time, you can put it in a pot and soak it in hot water or cook it.