The main purpose of studying the pendulum in this story is to provide a vehicle for finding and sorting out variables and designing a study for discovering the variable that controls the period (the time for a back and forth swing) of the pendulum. The physics of the pendulum is much too sophisticated for young children and will wait for later years. On the other hand, it will come as a surprise to most students, and possibly to adults, that the only variable that makes a difference in the period of the pendulum is the length of the pendulum. The period is defined as the time it takes for the pendulum to make one complete swing, forth and back.