Water molecules cling tightly together, and that pull — surface tension — keeps oil floating in a separate layer on top. The two stay apart no matter how long you wait. Add liquid soap, though, and it breaks that surface tension, letting the oil blend in with the water.

Soap molecules grab onto both water and oil at the same time. Water molecules cling tightly together — a pull called surface tension — which keeps oil floating in a separate layer on top. When you add liquid soap to a jar holding both water and vegetable oil, the soap breaks the surface tension and lets the oil blend in. The two layers that stayed apart now mix.

Oil and water are immiscible. They form separate layers instead of blending. You add vegetable oil to a jar of water and watch the two layers stay apart. Then you add liquid soap and stir. The soap lets the oil blend in. This proves that a special substance can force two immiscible liquids together.