You might keep your money in a bank, but have you heard of a time bank? This online service allows users to trade services with one another without exchanging money. For example, a hairdresser gives someone a haircut for one hour of credit in the time bank. The hairdresser could then use that credit to have a plumber come work for her for an hour. Time banks are useful for people who can't arrange direct trades. The plumber might not want a haircut, but he can work for the hairdresser and earn credit to use with someone else. How does the detail "The plumber might not want a haircut, but he can work for the hairdresser and earn credit to use with someone else" best support the central idea that time banks are a convenient way to get things done?