Piggy banks are typically made of ceramic or porcelain, and serve as a pedagogical device to teach the rudiments of thrift and savings to children; money can be easily inserted, but in the traditional type of bank the pig must be broken open for it to be retrieved. Most modern piggy banks, however, have a rubber plug located on the underside; others are made of vinyl and have a removable nose for easy coin access. Some piggy banks incorporate electronic systems which calculate the amount of money deposited.
This name may have caught on because the pig banks were mostly used by children, and the pig is a child-friendly shape that is easy to fashion out of clay. Once the meaning had transferred from the substance to the shape, piggy banks began to be made from other substances, including glass, plaster, and plastic.
Another reason for the name piggy bank that has been put forward is based upon the idea that the coins given to the piggy bank represent the food fed to a pig by the farmer. It costs the farmer money to feed the pig which he does not get back until the pig is slaughtered for the meat (represented by breaking the piggy bank) which the farmer can then sell.
In a curious case of parallel evolution, the Indonesian term celengan (a celeng is a wild boar, with the "an" affix used to denote a likeness) was also used in the context of domestic banks. The etymology of the word is somewhat obscure, but from a Majapahit piggy bank from the 15 century A.D. one may conclude that the concept has been used for several centuries.
In Seattle, Washington; The Pike Place Market's official mascot, Rachel, is a bronze cast piggy bank that weighs nearly 600 pounds, is located at the corner of Pike Place under the "Public Market Center" sign. Rachel was designed by local artist Georgia Gerber and modeled after a pig (also named Rachel) that lived on Whidbey Island and was the 1977 Island County prize-winner. Rachel receives roughly $9,000USD annually in just about every type of world currency, which is collected by the Market Foundation to fund the Market's social services. Locals make a habit of emptying their pockets and rubbing Rachel's snout for good luck.