Surface Tension

physics · liquids · water

Liquids have skin. Water and other liquids form an invisible layer on their surface that acts like a stretchy, elastic skin. This happens because molecules at the surface are pulled inward by the molecules below them, creating tension.

Water striders walk on water. Some insects can walk across water without sinking because surface tension creates a thin, strong film that supports their tiny feet. It's like the water is a trampoline they can bounce on.

Raindrops are round. Water droplets form spheres because surface tension pulls the liquid into the smallest possible shape, which is a ball. This is why raindrops and soap bubbles are round instead of flat.

Soap breaks the skin. Soap is special because it weakens surface tension, which is why it helps water spread and wash things. Without soap, water would bead up into little balls instead of spreading to clean your hands.