It's probably easier to see what is happening with the water and wine problem if you think about where each individual molecule is, rather than assuming the liquids are continuous. So to simplify it, do one of the sock experients; Imagine two boxes, one containing one hundred white socks and one containing 100 black socks. Take five from the 'black' box and put them in the white box. Then, eyes closed, mix them all about and without looking take five from the mixed box and put them back in 'black' box. Now, open your eyes! Are there more white socks in the black box or black socks in the white box? Obviously, they must be the same, since every sock is somewhere (unlike in real life!)and there are the same number of socks in each box as there was at the start.