Most household and personal care products contain one or more surfactants. Surfactants are characterized by ionic or polar “head groups” and hydrophobic “tails.” They serve to stabilize high surface area systems such as emulsions and foams by reducing surface tension. They can be classified as anionic, cationic, neutral, amphoteric, or zwitterionic based on charge. The classical example of surfactants is soap prepared by alkaline hydrolysis (saponification) of animal or plant fats and oils (oleochemicals). Beginning early in the 20th century, synthetic surfactants, often called detergents, were introduced. These can incorporate a variety of polar or charged head groups and the hydrophobic portions can be obtained from oleochemicals or petrochemicals. Formulation of specific products depends on the anticipated use. Laundry and dishwashing detergents usually function at elevated temperature and may contain materials for stain removal, including enzymes. Interaction with Mg2+ and Ca2+ ions present in “hard” water is detrimental and “builders” are used to complex these ions. Personal care products may use milder detergents and often include moisturizers (humectants) and oils intended to restore the water and fatty materials removed by the detergents. Shampoos are used with “conditioners” that provide a surface coating to hair to achieve smoothness and sheen. Toothpastes include abrasives, humectants, flavors, and colors. Personal care products may also include material with active biological functions, such as exfoliation, hair growth control, or antibacterial activity. Preservatives may also be present. While many of the materials used in household and personal care products originate in the chemical industry, the final products are usually distributed by companies specializing in consumer products. Advertising plays a key role in this segment of the economy.