Oil and water separate because water molecules cling to each other more than to oil.

The longer version

Water molecules are strongly attracted to one another, sticking together through hydrogen bonds. Oil molecules lack that attraction, so a water molecule surrounded by oil would have to give up those comfortable links. The water therefore squeezes the oil out and stays clumped with its own kind.

Shaking blends the two for a moment, but the droplets soon merge back together, and the lighter oil rises to the top. Density plays a part as well, since most oils weigh less per volume than water. Substances that can bond with both sides can bridge the gap and hold a stable mixture together.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.