This is an ideal experiment, means no change of volume is assumed. (In 'reality' it changes when two different liquids are mixed, for instance 50 mL of Alcohol plus 50 mL of water aren't 100 mL mixture!!!). But at this experiment one can safely ignore the effect. You have a glass of red wine and a glass of white wine. Both have exactly the same amount (volume) of wine. Now you take out one level spoonful of the red wine and pour it into the white wine, then you stir thoroughly. As next step you take out one level spoonful of the mixture and pour it back to the red wine and also stir thoroughly. Now think about is there more white wine in the red wine or more red wine in the white wine? And what changes if one would start with the white wine? After you have made your decisions, try to calculate the results (percent). It'll surprise you…or not, lol….