surface tension
Surface tension is the way the surface of a liquid behaves like a thin, stretched elastic skin. It is why water forms round droplets, why a dry needle can float on water, and why some insects walk across a pond without sinking. It answers the question: why does a liquid's surface seem to pull itself tight and resist being broken?
The precise cause lies in the attractions between molecules. Inside the liquid, each molecule is pulled equally in all directions by its neighbours. But a molecule at the surface has neighbours only below and to the sides, so it feels a net inward pull. The surface therefore acts to shrink itself to the smallest possible area, which is exactly why free droplets pull into spheres, the shape with the least surface for a given volume. The strength of this effect is the surface tension gamma, defined as the force per unit length along the surface, measured in newtons per metre (N/m); equivalently it is the extra energy stored in each unit of surface area.
Surface tension drives capillary action (water climbing up a thin tube or a paper towel), lets small insects like water striders skate on ponds, and shapes bubbles and raindrops. One honest clarification: surface tension is a property of the surface of a liquid, arising from molecular cohesion, and it is entirely separate from viscosity, which is about internal flow. Adding soap lowers water's surface tension, which is part of why soapy water cleans and forms easy bubbles.
A water strider stands on a pond because its water-repellent legs dimple the surface without breaking it; the surface tension of the water, about 0.072 N/m, supports the insect's tiny weight.
Surface tension makes a liquid's surface act like a stretched skin, pulling droplets into spheres.
Surface tension is a property of a liquid's surface, from molecular cohesion, and is separate from viscosity. Soap and detergents lower it, which helps water wet and clean surfaces.