Given the amount of mail that arrives during the holidays--from early December to early January--the post holiday season provides an excellent opportunity to perform an informal science and mathematics classification activity--determining where our mail is coming from. In the included activity, students extend their scientific thinking through the processes of classifying, observing, gathering, hypothesizing, and generalizing through the use of zip codes. Students will be astonished to learn that it is possible to determine the origin of a piece of mail by simply examining the zip code of the sender.