Two identical perfectly insulated containers are connected by a narrow tube that has a control valve. Initially, all the liquid is in the left container up to a height h (average height of h/2). When the valve is opened, the liquid flows from the left container into the right one, the system finally coming to rest when the levels are h/2 in each container (average height of h/4). The initial gravitational potential energy of the liquid is mg(h/2).The final gravitational potential energy is (m/2)g(h/4) + (m/2)g(h/4) = mgh/4. Half of the initial gravitational potential energy has disappeared! Why?