People drinking tea or coffee could leave the coffee or tea stains easily inside the cup. There is some cleaning detergents for that but I have used the cheaper option and I highly recommend you to use.
It’s Baking Soda! You can see the before and after effect below:
|Before and after (left to right)|
2) add a little bit of water (1-2 teaspoons)
3) use a kitchen wrap or paper to remove the stains
Have a try!