REALLY P1SSED: it is empty (I drank it)
AS TAUGHT IN ENGINEERING SCHOOL: it is twice as big as it needs to be.
It is always full.
It may be filled with 50% of water and 50% of air, or any variation of competing percentages, but it is always full of something even if it looks empty as it could be FULL of air.
It's completely empty until you put something in it.
The cup is half full. A glass can't be half empty because half of nothing is still nothing.
Was the previous condition of the glass full or empty?
If full, then the answer is half empty.
If empty, then it is half full.