And if there's nothing in it, you're a schizophrenic.

To title: Or you're just thirsty....

The glass is always full. Whether half of it is air or it's filled with water, the glass is always full.

Optimist: half full
pessimist: half empty
Realist: there is water in the glass

you could say someone who had everything and lost half would look at it as half empty and someone who had nothing and gained half would look at it as half full.

I thought I was the only one who thought like that

I have said this since I was 10. I guess I'm a realist.