It's rude to tip someone an extremely low amount.
Kenny just finished some drinks at the bar, and was served by a top-notch bartender.
Kenny wants to leave a gratuity.
Kenny pays for his tab and leaves the bartender a 20 dollar bill to keep.
Kenny digs into his pocket and takes out 25 cents for the bartender to keep.
The bartender is grateful for the generous amount and looks forward to the next time Kenny comes in.
The bartender is offended by the tiny amount and promises himself next time not to provide as good of service.