I have a theory. You know how 2-Liter soft drinks lose their fizz if you keep them for more than a day or so? My theory is that the the carbon dioxide leaking out through the now opened cap. But what if you turn the bottle upside down, so that the air doesn’t go out the top? The liquid seals the lid shut, keeping the carbonation in. Drinks stay fizzy for days.

So far, it’s worked for me.