I'm not much of a drinker, and I noticed that even though I'd put the cap on tightly, the level had dropped in one bottle. And what was left didn't seem any stronger.
Another bottle had never been opened, but the level there dropped too. It turned out that the paper or foil or whatever going 3 inches down the neck of the bottle had come off the bottle and it was evaporating without ever having been opened.
Neither cap would tighten until I squeezed it enough to distort it a little. Before that, the cap threads were bigger than the bottle threads.
Admittedly, it was years since I had drunk from one or gotten the other.
I think the lesson is, Drink more.