"Cheaper by the Dozen" is a book by Frank Bunker Gilbreth, Jr. and Ernestine Gilbreth Carey that tells the story of motion study and efficiency experts Frank Bunker Gilbreth and Lillian Moller Gilbreth, and their twelve children. The title comes from one of Gilbreth's favorite jokes: it often happened that when he and his family were out driving and stopped at a red light, a pedestrian would ask "Hey, Mister! How come you got so many kids?" Gilbreth would pretend to ponder the question carefully, and then, just as the light turned green, would say "Well, they come cheaper by the dozen, you know, " and drive off.