Centre of mass

I made this odd shape out of |Lego to test an idea about centre of mass. I have been teaching that the centre of mass is the point with zero resultant torque - ie the sum of the clockwise moments equals the sum of the anticlockwise moments. I balanced it on a knife edge to find this point.

 One problem that my class has had is that there are equal masses on either side of the balance point. This is not true. It is equal MOMENTS. So you can have less mass on one side provided it is further away so that it has a bigger moment. The results are shown below.

 They are clearly not equal parts!