DD - One day when my daughter was little, after picking her up from school I asked her how her day was. She responded with a heart-broken "it was horrid!" I asked her what made it horrid, and she told me about an incident. I asked her what amount of time that incident took, and she responded that it took about 10 minutes. I then asked what else happened during her roughly 9 hours in the day at that point. Everything else she experienced was fun, or at least neutral. I pointed out to her that 10 minutes of an unhappy incident should not be allowed to ruin an entire day - it was a matter of how she chose to perceive the incident - and in fact, the vast majority of her day sounded like it was a good one. She could choose to see the incidence as merely an interruption in her day, as something to get through and move on, and take the rest of the day for what it was. Fortunately, she grew up choosing to look at things differently after that.
Now sometimes catastrophies happen in the blink of an eye which really do ruin the rest of the day, but that's not what I see the glass half-full or half-empty thing as being about. I see it as being about things like the incident which occurred for my daughter that day. How we choose to deal with the normal ups and downs of life is just that - our choice. It's really about choosing to see either the positive or the negative in any given situation. If you can see both half-empty and half-full as a combination of "I just enjoyed half a glass, and I still have half a glass left to enjoy", then both are positive. What if you were simply given a half-filled glass, and did not create it's half-filled situation by enjoying the now-empty half? Would you then see it as half-empty (I did not receive half a glass), or as half-full (I received half a glass more than the nothing I had before)? And how would you see it if both you and your neighbor were each given a glass, but your neighbor's glass was full, while yours was half-filled?