Dish-soap contains surfactants—tiny helpers that break the milk’s surface tension. When surface tension is broken, the milk’s fat and water begin to flow, and they drag the food coloring along with them.

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Dish soap contains surfactants, which are tiny helper molecules that can break the surface tension of liquids. Surface tension is the invisible force that holds liquid molecules together at the surface, creating a kind of skin on top of the liquid. When surfactants from dish soap are added to milk, they immediately begin to disrupt the surface tension. The surfactant molecules insert themselves between the milk molecules, breaking the bonds that hold the surface together and weakening the invisible skin on top. When the surface tension is broken, the milk's fat and water molecules are no longer held rigidly in place. They begin to flow and move around freely, creating currents and movement within the milk as the molecular structure becomes more fluid. As the flowing fat and water molecules move through the milk, they drag the food coloring along with them. This creates the beautiful swirling and flowing patterns that we observe when soap is added to milk containing food coloring drops. To summarize what we have learned: dish soap contains surfactants that break the surface tension of milk. When this happens, the milk's fat and water molecules begin to flow freely, and they drag the food coloring along with them, creating the beautiful swirling patterns we see in this classic science demonstration.