Glass beads are placed in the compartments of a horizontal square grid. This grid is then vertically shaken. According to the reduced acceleration [image omitted] of the system, the granular material exhibits various behaviours. By counting the number of beads in each compartment after shaking, it is possible to define three regimes. At low accelerations, the grains remain in their compartment, and the system is frozen. For very large accelerations, the grains bounce out of the compartments and behave as a "binomial gas": the system is homogeneous. For intermediate accelerations, grains form clusters, i.e. grains gather in some particular compartments. In that regime, the probability for a bead to escape from a site depends on the number of beads contained in the concerned compartment. The escape probability has been measured with respect to the number of beads in a compartment. Above a given number of beads, the beads remain trapped in the compartment. A basic numerical model reproduces some of the results and allows us to explore the dependence on the initial conditions. (Contains 6 figures.)