surface tension
/ SUR-fis TEN-shun /
Carefully lay a steel paperclip flat on water and it floats, though steel is far denser than water. Or watch a single raindrop pull itself into a near-perfect sphere. The water surface acts like a stretched, invisible elastic skin — and the tightness of that skin is what we call surface tension.
Surface tension arises because a molecule deep inside a liquid is pulled equally in all directions by its neighbors, but a molecule at the surface has neighbors only below and to the sides, so it feels a net inward tug. The liquid responds by shrinking its surface to the smallest possible area, which makes the surface behave like a taut membrane. Numerically, surface tension is the energy needed to create a unit of new surface area, or equivalently the inward force per unit length along the surface.
Why it matters: surface tension shapes droplets, lets insects walk on water, drives capillary rise in plants and paper towels, and governs how detergents and lung surfactants work. The honest caveat is that it weakens as temperature rises (the molecules jiggle harder) and can be slashed dramatically by adding a surfactant, which is exactly how soap lets water wet greasy dishes.
A water strider stands on a pond without sinking: its waxy, water-repelling legs press dimples into the surface, and the surface tension of the water holds its weight like a trampoline.
Surface tension lets light objects rest on a liquid's surface.
Surface tension is not the liquid being 'stronger' at the top — it is the same intermolecular forces felt unevenly because surface molecules have fewer neighbors. Adding soap (a surfactant) lowers it sharply, which is why soapy water spreads and wets surfaces that plain water beads up on.