  In the Brazil nut problem (BNP), hard spheres with larger diameters rise to
the top. There are various explanations (percolation, reorganization,
convection), but a broad understanding or control of this effect is by no means
acheived. A theory is presented for the crossover from the BNP to the reverse
Brazil nut problem (RBNP) based on a competition between the percolation effect
and the condensation of hard spheres. The crossover condition is determined,
and the theoretical predictions are compared to Molecular Dynamics simulations
in two and three dimensions.
