The big idea
A clock stores energy and lets it out in tiny, equal pieces. The falling weight gives the push, the pendulum sets the beat, the escapement lets out one tooth at a time, and the gears count the beats into seconds, minutes and hours.
Memory trick: “Long is slow, short is quick; the length, not the width, sets the tick.”
A bit of history: As a student, Galileo is said to have timed a swinging lamp in the cathedral at Pisa against his own pulse. At the end of 1656 the Dutch scientist Christiaan Huygens designed the first pendulum clock. Clocks built to his design the next year kept time far better than any clock before them.