The balance of the glass half full or half empty as a metaphor for life is often used. (I never see the either or part of it). I see the change that takes place is like pool water or any clear liquid. Our experiences flavor the water, will it alter it from a pure form to an acid form? That is what life does, as we age our glass of half full water changes PH, sometimes acidic,sometimes alkali. Our experience will always have an effect on the water, but as long as we keep the level balanced we can still live a balanced life, it may take a little thought to turn it from a sour experience to a positive and sweet experience. At the end of the day we are the ones responsible for maintaining our balance.