At a mechanical instability in our usage, the stress drops suddenly and an elastic shock is produced. Instabilities on two very different scales are observed in the laboratory: those that are clearly audible and involve a stress drop of about 10 bars or more, and those of the sort studied by Mogi, Scholz, Byerlee, and others, which can only be detected with considerable amplification. The latter type are termed microfracturing events or microshocks. Earthquake-oriented studies in our laboratory are based on the assumption that in the earth significant earthquakes result from the first of these instabilities. This assumption may not be valid and needs study.