A cylindrical soda can has a radius of 6cm and a height of 12cm.
When the can is full of soda, the center of mass of the contents
of the can is 6cm above the base on the axis of the can (halfway
along the axis of the can). As the can is drained, the center of
mass descends for a while. However, when the can is empty (filled
only with air), the center of mass is once again 6cm...