has a cup in it and there is some tea in that cup.
If someone asks you in a cafe what you would like
to drink, you might well answer:
like a tea, please!
have, in other words, used the indefinite article
with a mass noun and got away with it. This is formally
correct despite what we have said so far. What is
happening here is that the phrase a tea is
shorthand for a cup of tea. The noun cup
is a count noun, but we have recategorised the mass
noun tea and given it a countable meaning,
i.e. it stands for the container that has the tea
in it. We do this all the time with drinks and glasses
and cups, etc.