Watch the colors flow and mix.
The colors move as the soap spreads across the surface of the milk. Once soap covers the surface, the swirling will stop. (If you use water instead of whole milk, the colors will start and stop in an instant.)
In whole milk, fat is the secret ingredient that keeps the colors moving. As the soap spreads out, it sticks to tiny globules of fat. As the globules take up soap, they make room for more soap to spread out.
By: Ed Pessalano
Art by: iStock.com/nayneung1