Money can be compared to the oil in the engine of the modern economy. Without a sufficient quantity of money, the economic engine 'tightens' and is often beyond repair; conversely, too much money can cause the engine to run away. In this case, the engine 'floods' and must be purged to run again. But while very few social institutions are as essential to the functioning of the economy as money, it is paradoxically very difficult to define. Certainly, the collective imagination associates money with precious metals (such as gold or silver) or, today, with lines of computer code.