I found a project where 1st graders recreated the whole process: http://www.lake-mills.k12.ia.us/elem/1st/sb-e2.html But here's what happens: After a letter is dropped in the mailbox, a mail carrier picks up the mail during his regular route. After completing their routes, carriers return to the post office with mail gathered from street collection boxes, homes, and businesses. Mail handlers, unload the sacks of incoming mail; separate letters, parcel post, magazines, and newspapers; and transport these to the proper sorting and processing area and then taken to other workrooms to be sorted according to destination. Clerks operating electronic letter-sorting machines push keys corresponding to the ZIP code of the local post office to which each letter will be delivered; the machine then drops the letters into the proper slots. The machines also have optical character readers (OCRs) and barcode sorters that can "read" the address and sort a letter according to a code printed on the envelope. Other clerks sort odd-sized letters, magazines, and newspapers by hand. Finally, the mail is sent to local post offices, sorted according to delivery route, and delivered. Reference Link: http://federaljobs.net/psclerks.htm