http://en.wikipedia.org/wiki/Shopping_cart Electronic and magnetic Electronic systems are sometimes used by retailers. Each shopping cart is fitted with an electronic locking wheel, or 'boot'. A transmitter with a thin wire is placed around the perimeter of the parking lot, and the boot locks when the cart leaves the designated area. Store personnel must then deactivate the lock with a hand-held remote to return the cart to stock. Often a line is painted in front of the broadcast range to warn customers that their cart will stop when rolled past the line. Unfortunately these systems are very expensive to install and although helpful are not foolproof . The wheels can be lifted over the electronic barrier and/or pushed hard enough that the locks break.[13][15] Some cities have required retailers to install locking wheel systems on their shopping carts. In some cases, electronic systems companies have encouraged passage of such laws to create a captive audience of potential customers.[16] Some trolleys have magnetic locking wheels. The supermarket floor has a magnetic strip around a designated perimeter line, if the trolley passes over this line the wheels lock and the trolley is rendered immovable. However the wheel locks can be broken and the trolley is movable again.