In this talk, we will discuss non-holonomic system of two balls rolling on each other, with the no-slip and no-spin condition, which defines a rank 2 distribution. Depending on the ratio of the radii of two balls, the symmetry group preserving that distribution may be non-trivial. In particular, if the ratio is 1:3, the symmetry group turns out to be the exceptional Lie group G2. The main results obtained will be presented, and based on the works by G. Bor and R. Montgomery.