This is one of those easy yet startlingly beautiful things that can be done with items you already have: milk, food coloring, and dishwashing liquid.
First, pour a glass of milk into a wide container. I used a cafeteria tray and whole milk (this is what the kids drink so this is what we have on hand and I would recommend whole milk because low or nonfat milk might not work as well). Next, I used chopsticks to spread the milk to cover the whole tray.
Second, let the kids squeeze dots of food coloring (we used the four colors that comes in the box) onto the milk. I had to help Zack because he has fat baby fingers.
We have kid chopsticks and they used them to draw lines and swirls in the milk. The design is better if they use lines to run the colors into each other rather than vigorously stir the colors together.
And here’s the exciting part of the project. Squeeze in one drop of dishwashing liquid (we used Dawn) and see what happens.
The kids start messing with it with their chopsticks so you can’t see all the exciting things that happens. It has to do with the dishwashing liquid breaking up the fat in the milk. Here’s a video that shows and explains it better (if you don’t have Flash, click on the picture and it will take you to the video on YouTube):