A well-known party trick is that for certain volumes it is possible to balance a partially full North American beverage can on its beveled edge. The "fluid level" slider controls the height of the top surface of the fluid and the point shows the center of mass of the fluid and can. The system will be stable if the center of mass is over its base of support. Thus if the point is between the vertical lines the can will balance on its edge.
The system is stable when it contains between about 45 and 155 ml of fluid (assuming a density of 1 gr/cm3). Experiments give an interval between about 40 and 200 ml. The difference is likely caused by our approximations of the geometry of an actual beverage can.
Rotate the figure so that you view the fluid level from the side (edge on). If you move the slider you will notice that the vertical component of the center of mass is at its lowest point when the center of mass is at the fluid surface. Students should ask themselves why this happens.