A shock in economics is defined as an unexpected or unpredictable event that affects an economy, either positively or negatively. Recessions are often modelled as negative economic shocks in which the current state of the economy is untenable and the economy tries to restore itself to a new equilibrium position. In the short-run, as the economy adjusts to the shock but before it reaches the new equilibrium, the shock often causes productivity to fall, unemployment rise, and closure of firms that are now non-viable in the new environment. The new environment enables new kinds of businesses and people must learn new skills and exploit new opportunities to achieve long-run equilibrium.