In an RFID system, information is encoded on a “tag” that contains a microchip and an antenna. The information can be accessed by a reader, which passes the data along to the person or system that needs it. Library tags are unobtrusive, usually a couple of inches square and very thin. They are also passive: they have no batteries or other power source. The energy needed to power and read the tag comes from the reader, which emits a signal that searches for a tag within its limited (about 18 inches) range. When the signal hits a tag, the tag “awakens” and responds with the requested information. When RFID is used for security purposes, the reader (which is located near the exit) asks the tag if the book has been checked out.