Surface tension is the surface energy between a liquid and its own vapor, or air, or a metal surface. The word tension comes from the force that resists any attempt to increase the surface area. Surface tension is thought to be a factor in the ability of an oil to "wet" a surface, in emulsion stability, and in the stability of dispersed solids. However, "wetting" has been found to be a complex phenomenon involving oleophobic and oleophilic films on the metal surface. Some additives markedly change surface tension. An example is water containing soap for the formation of bubbles. Silicone is added to mineral oils to reduce surface tension and as result, foaming characteristics may change.