Stores use a number of strategies to reduce shoplifting, including storing small, expensive items in locked glass cases; chaining or otherwise attaching items to shelves or clothes racks (particularly expensive items); attaching magnetic or radio sensors or dyepacks to items; installing curved mirrors mounted above shelves or video cameras and video monitors, hiring plainclothes "store detectives" and security guards, and banning the bringing in of backpacks or other bags. Some stores have security guards at the exit, who search backpacks and bags and check receipts. Stores also combat shoplifting by training employees how to detect potential shoplifters.