For a really big bang , the third factor is essential: leverage. This can be either in the market being considered or in other markets but employed by the same investors-hence overall economy-wide leverage is a danger even in a relatively non-levered market. Bank credit is a form of leverage: a banking system creates money by lending more than it has; and banks and other forms of credit are st the centre of a developed market economy. Credit creates value by enabling other factors of production to be employed.