well I guess you need to work out what will get you the response you want
personally I would either make a joke of it - or start drinking water
seriously there is not a lot you can do is she saying it jokingly or seriously
you could always look a little sad and say 'why would you say that? now I might have to start drinking bottled water - unless of course you want to go get the coffee for me'
dunno I guess you have to work out how it 'feels' - the energy/atmosphere at the time
or go to another coffee shop??
and see what happens when you come back with coffee from somewhere else?
