Coffee and Tea

a: Oh, hi! I'm going to buy present for my sister.
    I think tea is better. She and I do love drink a kind of tea.
    But I can't choose what tea is better.
b: Do you? I think... green tea is better.
    Because, coffee has so much caffeine.
    And it's just my opinion, I think green tea or black tee has less caffeine than coffee, aren't teay?
a: Yes, they are. Actually I think so but caffeine is not always bad thing. It's efficient in some cases.
    For example, when you have to finish your work until tonight, caffeine makes you concentrate it, isn't it?
b: Yeah, I agree. But sometimes caffeine offends me, too.
    So much Caffeine will make you nervousm, isn't it?
a: Ok, but, green tea and black tee have much caffeine, too.
    I don't know exactly about that, coffee and tea have different caffeine in it.
    So.... I will buy green tea. Because black tea makes my teeth colored.
    If I grin at somebody I like with coloring teeth.... oh my god.... Can you imagin that, don't you?
b: Yeah, Nice choice!