High-tech, full-body scanning machines are already in use at a handful of airports, but they may become more common as security officials around the world respond to the attempted attack on an airliner on Christmas Day. The Netherlands announced Wednesday that the scanners would be used for all flights heading from Amsterdam's airport to the United States, and an official in Nigeria later said that country planned to buy full-body scanners too. What exactly are these machines, and how do they work? What are the drawbacks to using these types of scans? Here are some questions and answers about the devices.