In figure molecules present in the lower part experience balanced forces as compared to the surface molecules. There is a inward force on the surface water molecules, this force causes surface tension.
Surface tension is also responsible for the shape of liquid droplets. Droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical because the sphere has the smallest surface area for its volume and this is due to surface tension.
This can also be understand in term of energy as, a molecule in contact with a neighbor is in a lower state of energy than if it were alone. The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors (compared to interior molecules) and therefore have a higher energy. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized quantity of boundary molecules results in a minimized surface area.
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