Here is the original question: - Well, the answer that I came up with is, that, since there is no eternal force on the system, the centre of mass cannot be displaced, no matter what. Now, my doubts are regarding this itself. When the ice melts, it turns into water of lesser volume. In the absence of gravity, the cube of ice turns into a cube of water, but the water does not spread out evenly on the tray. In this scenario, how is it possible that the water, whose volume is bound to be lesser than that of the ice, still maintains the same position of the centre of mass? If the volume is different, the geometry also has to be different to maintain the same position of the ice. How does it all happen?