The Mixing Problem

Say you have two barrels, one containing wine and the other containing an equal amount of water. You take a cup of wine from the wine barrel and add it to the water. If you want, you can stir it some — or not, it doesn't matter. Then take a cup of that wine/water mixture and return it into the wine barrel. The cup should hold the exact same amount so that both barrels once again contain equal volumes (but now mixed). The question is: which one of the mixtures is purer?

Thought about it? The answer is that both mixtures are equally pure! (← click to spoil)