I typically only drink herbal tea or green tea, but I had started drinking some rooibos tea and for the first time ever in all my daily tea drinking the inside of my mug got tea stains (see pic below). I would wash it really well with dish soap but to no avail. So I asked my mother about this and she said that baking soda does the trick! I mixed a little baking soda with water and scrubbed away, had to repeat a couple times, but it came over pretty easily!