Shaving cream is an example of a complex fluid.  Without stress, the foam appears to be a solid: it does not flow and can support (very) light  loads.  However, when adequate stress is applied, shaving cream flows easily like a fluid.  On the level of individual bubbles, the flow is due to rearrangements of small collections of bubbles.  On this scale, the flow is not smooth, but instead consists of fluctuations due to rearrangements of the bubbles and releases of stress.  These fluctuations are similar to the fluctuations that are studied in earthquakes.