So, it's another one of those age-old questions: is the glass half-full or half-empty? It's supposed to be a way to tell if you're a pessimist or an optimist... There are, however, flaws in that system. See the photo to the left. Mathematically, technically speaking, the glass is always 100% full. It was never specified which substance was being identified and measured. So, does that make someone a realist? Most people would say that makes them a smartass. I think it means that they're analytical. What about people who say it's half full? Maybe they're thirsty. Maybe they are thinking in terms of potential capacity. That could also be potential capacity to muck things up - it's possible to spill what's left, to somehow taint what's in there, etc. These are the things that would go unnoticed in a "half full" answer, incorrectly leading one to come to "ah, an optimist!" conclusion, nevermind the horrible possibilities that are chugging through the respondent's head. As for "half empty," that person may be thinking of where to fill the glass, or what they would do with the remaining liquid, hence making an optimist out of a seeming pessimist.
A true realist, I think, would bypass the smartassery of "well, technically it's completely full," the "half full" or "half empty" (no matter what the background dialogue) and ask a more important question - is the liquid drinkable/what is in said glass? To me, I think that if it's full of sulfuric acid, that thing is half full of death and get it away from me, quickly. I think that if it's used cooking oil, I don't give a crap how full or empty it is - I have no use for it. If it's water, and if I'm not too thirsty, sure - gimme some of that half full awesomeness. You get my point. The only point at which I think I would say it was half EMPTY is if I had myself consumed the missing portion. Otherwise, hey... Half of something you didn't have a second ago is plus one half.
And that's not optimism, that's common sense.