  Molecular Dynamics simulations of high density hard sphere fluids clearly
show a breakdown of the Stokes-Einstein equation (SE). This result has been
conjectured to be due to the presence of mobile particles, i.e., ones which
have the propensity to "hop" distances which are integer multiples of the
interparticle distance. We conclusively show that, even though the whole liquid
violates the SE equation at high densities, the sedentary particles, i.e., ones
complementary to the "hoppers", obey the SE relationship. These results
strongly support the notion that the unusual dynamics of fluids near
vitrification are caused exclusively by the presence of hopping particles.
