These lectures illustrate laboratory experiments investigating a spring-slider system shearing a granular bed. When the shear rate is low enough, the dynamics is intermittent, displaying a chaotic stick-slip motion which can only be described statistically. This is an instance, among many others, of systems exhibiting intermittent and erratic activity, in the form of avalanches characterized by self-similar fluctuations of physical quantities in a wide range of values. In analogy with equilibrium systems, it is thought that such properties originate from the vicinity of some critical transition, and therefore that systems microscopically very different could display similar and universal statistical properties. Investigation of such systems is thence not only interesting in itself, but can be of help in understanding the dynamics of a wider class of phenomena and in devising effective models.