Liquid surfaces behave like a stretched membrane, meaning they tend toward surface area minimization. This property is called surface tension. Molecules inside the liquid experience attractive forces equally on all sides from the surrounding molecules. But, surface molecules experience asymmetric forces. The net inward force on surface molecules pulls them down, causing surface tension. Work is done to move bulk molecules toward the surface, increasing the surface molecules' potential energy. This additional energy is surface energy. Consider a frame with a sliding wire dipped in soap solution to form a bubble with top and bottom surfaces. The surface tension acts inward perpendicular to the sliding wire, and external force is required to pull the wire outward. This force per unit length gives the surface tension. If the force pulls the wire through a distance, increasing the surface area, then the surface energy is given by the work done. This shows that surface tension can also be defined as the surface energy per unit surface area.