An array of systems can get jammed: traffic jams, compaction of granular materials, the glass transition, colloidal gels, panic and stampedes of people, foams, &ellip; The question that arises is whether all these systems share their features in a more fundamental way, enabling us to think of a jammed system as a new state of matter. Jamming could then be amenable to be described as a phase transition. A particular experiment aimed at understanding this phenomenon is presented here. It consists of a silo filled with grains, with an orifice in the base. Grains begin to pour freely, and then — if the orifice is not much larger than several grain diameters — the flow stops because an arch is formed: it jams. By measuring the number of grains between successive jammings (an avalanche), a measurement of the jamming probability is obtained. From the features of the statistical distribution of avalanches, some interesting characteristics of this process can be grasped. In particular, the probability density function of avalanches shows a clear exponential tail, therefore the system is not critical. It can be rescaled with only one parameter, namely the mean avalanche size (or, alternatively, the jamming probability of a single grain). We pay particular attention to the study of some variables that could, in principle, influence the process in order to ascertain which are relevant to the phenomenon of jamming, and we show the existence of a critical radius for the orifice above which no jamming can occur. © 2004 American Institute of Physics