and mobility of the weight again being indicated by the size and position of dots e g 58 or crossed lines e g 60 referring to fig7 the blocks could also be spherical types 70 72 74 76 each containing a smaller weight types 70 72 have markings comprising light and dark shading to indicate that they have their weights mounted in the center and that the weights are respectively lighter and heavier type 74 is marked with dark shading at one sector to indicate that a heavier weight is located there fig8 shows other examples of ways of stacking the blocks some rectangular blocks 80 can be made twice as long as other blocks 82 to build stack 84 the weight in block 80 must first be moved to the end 86 by holding block 80 with end 86 down to allow the weight to slide into position at end 86 the blocks can include types having other proportions sizes and external shapes more than two different masses can be used in different types of blocks the weights can have other fixed locations other techniques can be used for imparting an uneven mass distribution a different code for marking the blocks can be applied for example color coding instead of external markings the blocks can.