The glass is always full. The tricky part is figuring out what it's full of.
The running joke about the professor filling the glass with rocks, then again with pebbles, then again with sand, then again with beer, always asking if the glass is full, and always the students say yes.... The fictitious(?) professor ignores the fact that the glass *is* always full, and that he's always displacing something (air) with something else to "fill" the glass.
Of course, the student response that "there's always room for beer" works too :-)