Walking Water

We set up an interesting science experiment with two cups of coloured water - one blue and the other yellow.  A piece of paper towel hung from each down into a third empty cup.  We noticed that soon - the coloured water began to "walk" or soak into the towels and it wasn't too long before the liquids from both cups dripped into the empty cup and ... you guessed it - they mixed together to form a green colour!