/kloc-In the 6th century, metal coins circulated in the British market. After a long time, people found that full-value coins can be used as well as under-value coins. Therefore, people store full-price coins (good coins) and quickly spend coins with insufficient value (bad coins). As a result, bad money drove good money out of the market. This is Gresham's Law, a natural and social phenomenon of "survival of the fittest".