Deep inside a liquid, molecules are pulled equally in every single direction. Surrounding molecules attract each other with equal strength from all sides. However, the molecules directly at the surface face a different situation.
They do not have identical liquid molecules resting comfortably above them. This lack of upward pull creates a strong net inward force. This inward pull creates a constant state of tension across the surface.
The tension naturally forces the liquid to shrink its overall surface area. This shrinking effect explains why tiny liquid droplets form perfect spheres. A sphere has the smallest possible surface area for a given volume.
Surface tension depends heavily on the specific types of intermolecular forces present. Liquids with stronger intermolecular forces will have significantly higher surface tension. For instance, hydrogen bonding creates very strong surface tension in water.
A common misconception is that a physical skin exists on the water. In reality, it is just water molecules pulling tightly on one another. Heating a liquid gives its internal molecules much more kinetic energy. This extra movement disrupts the attractive forces and lowers the surface tension.