The deceptively simple question sets off a thorough, precise explanation of the workings of spring and weight clocks (i.e. an alarm clock and a grandfather's clock), electric and electronic clocks. The book looks and sounds rather like a Zim manual; if it's not a simple as the title or the format would indicate, it's as clear as the subject allows. Proceeding slowly and carefully from the storing of energy by winding, it examines the interlocking system of wheels and pinions, the function of balance wheel and pendulum in assuring the correct time, lastly how the hands are turned. Sharply drawn, neatly labeled diagrams appear where they're needed. Mr. Johnson is a collector and amateur horologist and his text has been appropriately vetted; with his book in hand, kids can begin taking clocks apart with confidence. It also appears to be the most detailed on the subject.