<p>Why would a supermarket exchange a piece of paper decorated with coloured squiggles and a picture of the monarch or a 'touch & beep' contactless plastic card for a basket of groceries? Money is a trick of confidence and there have been well over one hundred occasions since the French Revolution in 1789 that a currency has lost the population's confidence in its value and become worthless. It is important to understand what money is - and isn't...