The brief links between water molecules
A water molecule’s slight charge imbalance lets it attract nearby molecules, but the attraction is strongest when a hydrogen on one molecule points toward the oxygen of another. This directional attraction is called a hydrogen bond. It is much weaker than the bonds holding each molecule together, and it continually forms and breaks as molecules move. Even so, a molecule in liquid water is surrounded by many neighbors, so the combined attractions help keep the liquid cohesive.
Why the surface resists being stretched
A molecule deep in the liquid has neighbors on every side. A molecule at the surface has no water molecules above it, so its surroundings are uneven. Pulling more molecules to the surface means disrupting some of their attractions with neighbors. That takes energy, which is why water tends to keep its surface area small.
The effect is called surface tension: the surface behaves as though it were a taut film, though it is still liquid, not a literal skin. It helps explain why a small water drop rounds up—the shape minimizes exposed surface—and why some tiny insects can stand on still water without breaking through. A disturbance or added weight can deform that surface, but the molecular attractions make it resist at first.
