Surface tension is the tension of the boundary layer of liquids to its surroundings (other liquids, vapor phase or air). In the bulk of the liquid, equal forces are pulling each molecule in every direction, forces, that are exerted by adjacent liquid molecules. Inside the liquid, the result is a net force of zero. Only at the boundary layer of the liquid, where there are not molecules on all sides, the molecules are being pulled inwards. In the quest for achieving the smallest potential energy, liquids aspire to minimize their surfaces. Therefore, the preferred shape of the liquid is a ball in spherical shape, since a ball, at the same volume, always has the smallest surface area when compared to other shapes.