What is used to help water wash away greasy dirt?
What did you use this morning when you had a shower?
A soap is the salt of a long-chain fatty acid. These have the property that they will interact with greases and oils (i.e. interaction between the hydrocarbyl chains), and also with ions, (i.e. the carboxylate head). Because they are not effectively solvated by water, the soap molecules form micelles, entrapping grease and dirt. Detergents do the same thing.