For the majority of human history, people used their own muscles to provide almost all of the work required for survival and development. A thin slice of humanity achieved a moderate amount of personal comfort and leisure because they were able, often through an accident of birth, to control a portion of the daily work output of hundreds to thousands of their fellow humans. The only sources of work-in the engineering sense-that were not either human or animal muscle came from capturing falling water or intermittently by capturing the breezes through devices like cloth sails or wind mills.