I have done this craft before a few years ago and now that my youngest can actively participate in such experiments I decided to give it a go. My eldest was too young to remember it.
I gave them a bowl with milk a few drops of blue food colouring in it. Each got a food skewer and a little bit of dish washing liquid in a separate container. You dip the skewer in the dish washing liquid and then dip stick in the milk and watch how the food colouring moves in all direct away from the skewer.