This Demonstration illustrates a key experiment of Galileo that furthered the understanding of inertia and the conservation of energy. He blocked the wire of a pendulum in the middle of its trajectory, altering the pivot point. He observed that the pendulum bob always reached the same height as at the start. In modern terminology, at the bob's initial position, it has only potential energy; at its lowest position, only kinetic energy; and in between, there is an interchange between kinetic and potential energy, with a constant sum.