I noticed this intriguing phenomena whilst messing about in work with a perfect glass hemisphere the other day: Place the hemisphere on a surface, with the flat surface facing upwards. (Looks like a 'D' rotated 90 degrees clockwise). When I spin the hemisphere fast enough, it will spin until it is spinning on it's side (Looks like a 'D' rotating around the straight line in the 'D' ) Then it will 'tip' over the edge, and by the time it stops moving, the flat surface is facing down ('D' rotated 90 degrees anti-clockwise) It appears any force which I exert which is not purely rotational about the primary rotation axis grows until the 'wobble' is enough to topple the hemisphere. The only possible explanation I can come up with myself is that the friction force with the surface is influencing the motion. If someone could shed some light on this I would appreciate it!