Bubble Bath Day is celebrated today. A bubble bath is a filled bathtub with a layer of surfactant foam on the surface of the water and consequently also the surfactant product used to produce the foam. Bubbles on top of the water, less ambiguously known as a foam bath, can be obtained by adding a product containing foaming surfactants to water and temporarily aerating it by agitation.
The practice is popular for personal bathing because of the belief that it cleanses the skin, that the foam insulates the bathwater, keeping it warm for longer, and prevents or reduces deposits on the bathtub at and below the water level produced by soap and hard water. It can hide the body of the bather, preserving modesty or, in theatre and film, giving the appearance that a performer who is actually clothed is bathing normally. Children find foam baths particularly amusing, so they are an inducement to get them into the bathtub.
The earliest foam baths were foamed with soap, which practice came about shortly after soap flakes were marketed. Saponins were also used to foam machine-aerated baths. Foam baths became more popular with later surfactants. Foam baths became standard practice for bathing children after the mass marketing of products so positioned in supermarkets during the 1960s. (from cute-calendar.com and Wikipedia)