Surfactants cleanse and build foam by acting at the surface between fat and water (surface-active agents or surfactants). They are able of being mixed with water and fat of the skin, allowing dirt to be removed. Based on their cleansing power surfactants are classified into primary and secondary or co-surfactants. Surfactants form the base of all personal cleansing products and can also have wetting, conditioning, defatting, emulsifying, & thickening effects. Surfactants are useful for homemade soaps (particularly liquid soaps) to improve lathering and decrease harshness.