Why is a piggy bank a piggy bank?

During The Middle Ages, in about the fifteenth century, metal was expensive and seldom used for household wares. Instead, dishes and pots were made of an economical clay called pygg. Whenever housewives could save an extra coin, they dropped it into one of their clay jars.They called this their pygg bank or their piggy bank. Over the next two hundred to three hundred years, people forgot that "pygg" referred to the earthenware material. In the nineteenth century when English potters received requests for piggy banks, they produced banks shaped like a pig. Of course, the pigs appealed to the customers and delighted the children.