When you pick it up to take the first sip, the hope is that it will wash across your palate, leaving you with a perfectly pleasant experience, perhaps a warm glow. Take a few more sips just to savour the feeling.
So the glass is back on the table, and maybe only half of it still remains. Your heart and soul will know only the feeling, but your eyes will tell you that not all is well. The sudden awareness of the state 'only half left' is so finite.
Now is not the time to think about that because, when next you pick up the glass, the very last thing that you will want to do is to cry into it.