Surface tension is a physical phenomenon of liquids that acts as if the surface of the liquid has a skin or membrane. Surface tension is the reason some insects can walk on water, and why bubbles and droplets have a spherical shape. It arises from the cohesion of water.

A paper clip on the surface of water.

Cohesion is the property of water where water molecules are attracted to each other. Deep into the body of a liquid, such as a lake, water molecules are surrounded by other water molecules and form hydrogen bonds in all directions. However, the water molecules at the surface of the lake are not entirely surrounded by other water molecules; there is an interface with the air. This means that the surface water molecules have fewer neighbors to form hydrogen bonds with than the water molecules deeper into the lake. The surface water molecules then form additional hydrogen bonds with their neighbors, so the cohesion between neighboring water molecules is stronger. Hence the origin of surface tension.