How to get rid of tea stain from thermal mug?

I have a habit of drinking green tea everyday (working days only, hehe..) using my thermal mug, can keep warm mar.. drinking is good, but the tea stain makes me feel very headache, the whole mug looks so dirty, yellowish!! and to clean them is not easy, you need to scrub (i also dono whether can scrub or not. but i cannot tahan the tea stain, so dun care lar...) real hard to get rid of those tea stain..and it will come back again after 2 weeks, no matter how u wash everyday.

I decided to google about it and i saw some ppl suggested that used lemon juice or baking soda to clean it. Well, not really sure wat is baking soda, but i have lemons in my fridge!! haha, it's experiement time!! i cut lemon into half, 'hmm seh dak' to use the whole lemon. hehe.. squeeze it and pour it into my thermal mug. then i add in hot water and soak over night.

the next day i checked, hmm, although it's not 100% clean, but it does helps, perhaps i need to use the whole lemon la.. hehe... i should try baking soda next time, see which one is better!!