Care for some Iced Tea?

Empty is a strange word. Nothing is ever really empty, everything is filled with something. So why does a word like empty exist? It seems unnecessary to have a word for something that is impossible. Is something empty because it does not contain something we want? Say you have a glass full of ice cold tea and by some means you have no more tea. Maybe you drank it, maybe someone else did or maybe it spilled. However your glass came to be without ice tea some would say it is empty, but no air has taken its place, air is something, it can't quench your thirst, but it is still something. So it is not empty. You could go fill your glass up provided there was more iced tea, but if not and ice tea was all you wanted well you are out of luck and left with air.
So when you feel empty, it actually doesn't mean you feel nothing, you just don't feel what you want and if you can't fill up on what you want, you are left with the equivalent of emotional air. What is that? Sad? Lonely? Want?