Watching the history of soap I learned that soap was scarce in Germany during WWII. The vegetable oils used to as part of the soap making process were hard to get. Detergent was invented in WWII by German scientists when they tried to create artificial soap. Soap leaves a scum on things detergent doesn't.

Somewhat related to the article is suds and salt. Public fountains that are vandalized with detergent can prevent it by adding salt to prevent suds from forming.