Pour a little milk onto a plate. Pour a small amount of water onto a different plate. Add drops of food coloring around the edges of the plates. In the center of the plate add a few drops of dish detergent. Notice what happens on each plate.
Questions to ask:
What happened on each plate? (The colors spread out and reacted differently to the detergent).
What does milk have in it that water does not? (The milk has fat in it).
What did the detergent do? (The detergent tries to break down the fat and the colors spread out differently).