Here's me answer to that age-old question: the glass is half-empty. Whether or not that makes me a pessimist is irrelevant, because me answer is based on statistics!
Whether or not a glass if half-choc-a-bloc or half-empty depends on whether it is in the process of being filled, or emptied. If a glass is being filled, when it reaches half-way then it's half-choc-a-bloc. If it's being emptied, when it reaches half-way then it's half-empty. Make sense?
So how do we know whether the glass is being filled or emptied? Well we don't. But we can work out the average based on (admittedly unscientific) statistics...
How are glasses filled? In the vast majority of cases it's by pouring liquid into them. And how long does it take to pour liquid into a glass? Let's say anywhere between 1 and 10 seconds, to be safe. So how are glasses emptied? Well in the majority of cases it's by drinking the liquid. So how long does it take to drink liquid from a glass? This can be anywhere from a few seconds to a few hours (reckon of someone sitting at a bar or a tea party).
If we pressed pause on the world and then labelled every glass as either 'being filled' or 'being emptied', I reckon ya'd find that the majority of glasses were in the process of being emptied. This is because emptying a glass takes, on average, considerably longer than filling it.
Therefore we can say that, on average, the glass is half-empty!