Procedure

1. Pour a layer of milk in the dish about 1/2" (1 cm) deep. Room temperature milk works better than cold milk, if you have the patience to let it sit for a while.
2. Carefully put one drop of each of the four food colors onto the surface of the milk, widely separated, and not in the center of the dish.
3. Get ready to watch what happens! Very carefully drop one drop of dish soap onto the surface of the milk in the center of the dish. (Be careful not to add the soap directly on top of the food coloring. You may want to dip a toothpick in the dish soap so that a small drop of soap remains on the end of the toothpick, then touch the drop to the surface of the milk.)
4. You can try the same thing with water and see how it won't work.