These are
dice which can be used anywhere a normal
six-sided die is required. Each die is a
sphere with the numbers 1 to 6 printed evenly on the surface. Inside, the
die is
hollow and contains a small
ball bearing which falls into one of six pits. These pits are aligned with the numbers on the surface, so the weight of the ball bearing will cause one number to remain facing upwards.
Of course, the idea isn't limited to only six-sided dice. However, adding more "sides" makes
construction of such a die much more
complex.
Spherical dice have no particular
advantage over regular dice, and take forever to stop
wobbling.