If you buy a tub of yogurt (one that has 4 servings), then eat some, then put it in the refrigerator for later.... then the next day or whenever you come back there is a small layer of liquid on top of the yogurt. What is this liquid? Should I dump it out or eat it? What exactly is it?

I have noticed this on a few different brands (though I am currently eating Mountain High brand nonfat vanilla).

There's no problem in eating that. A similar effect occurs to sour cream and cottage cheese. Its just the natural seperation of solids and liquids over time. Stir everything back together and enjoy.

I believe that liquid is a form of whey. And while it doesnt taste great (most people pour it out I think), I eat it... :)