Do you object to drinking black tea , Mr.Zhang ?   Yes , that is why I never drink it .   Do you drink green tea , then ?   Very seldom , because tea has tannin which may be harmful to one's health .   But if the tea is good and well prepared , it will do much good .   What good is it for ?   Not only can it appease one's thirst , but it can also refresh one when tired .   That's true , but it must not be allowed to stand long .   Why is it so ? 