Gwen is considering hiring a painter to paint her house. She runs an advertisement in a local newspaper seeking a painter. Rico answers the advertisement and visits Gwen's house. Rico says he might be able to paint the house this weekend. Gwen promises to pay him $1,000 if he does. Rico arrives that weekend and begins painting. What type of contract do Gwen and Rico have, and can Gwen tell Rico she has changed her mind once he has started to paint?