Electronic items, such as laptop computers , have so many different items packed into a relatively small area that it can be difficult to determine if a bomb is hidden within the device. That's why you may be asked to turn your laptop or PDA on. But even this is not sufficient evidence since a skilled criminal could hide a bomb within a working electronic device. For that reason, many airports also have a chemical sniffer . This is essentially an automated chemistry lab in a box. At random intervals, or if there is reason to suspect the electronic device that someone is carrying, the security attendant quickly swipes a cloth over the device and places the cloth on the sniffer. The sniffer analyzes the cloth for any trace residue of the types of chemicals used to make bombs. If there is any residue, the sniffer warns the security attendant of a potential bomb. In addition to desktop sniffers like this, there are handheld versions, that can be used to "sniff" lockers and other enclosed spaces and unattended luggage. Walk-through models, such as GE's Entry Scan 3, are also available. These sniffers can be used to detect explosives and narcotics.