PHYSICS

Look closely at a pond and you might see tiny insects skating across the top as if the water were solid. That trick is surface tension: the tendency of a liquid surface to behave like a stretched, elastic skin. It comes from forces between the liquid's own molecules, not from a separate film floating on top. Inside the bulk of the water, each molecule is pulled equally in all directions by neighbors. At the surface, molecules lack neighbors above them, so net forces pull them inward and sideways. The surface contracts to the smallest area it can, which is why droplets turn spherical and why you can float a paper clip if you place it gently.