THE idea of self-organized criticality(1) (SOC) is commonly illustrated conceptually with avalanches in a pile of sand grains. The grains are dropped onto a pile one by one, and the pile ultimately reaches a stationary ’critical’ state in which its slope fluctuates about a constant angle of repose, with each new grain being capable of inducing an avalanche on any of the relevant size scales. Some numerical models of sand-pile dynamics do show SOC1-8, but the behaviour of real sand piles remains ambiguous(9-18). Here we describe experiments on a granular system--a pile of rice-in which the dynamics exhibit self-organized critical behaviour in one case (for grains with a large aspect ratio) but not in another (for less elongated grains). These results show that SOC is not as ’universal’ and insensitive to the details of a system as was initially supposed(1), but that instead its occurrence depends on the detailed mechanism of energy dissipation.