Simplify the following text for a Grade 4 native English-speaking person.
Original: Piggy bank ( sometimes penny bank or money box ) is the traditional name of a coin accumulation and storage receptacle ; it is most often , but not exclusively , used by children . The piggy bank is known to collectors as a `` still bank '' as opposed to the `` mechanical banks '' popular in the early 20th century .
Simplified: We are not certain about why it is called piggy bank and how it started .