Travelers still getting used to removing their shoes in public and pouring shampoo into tiny bottles may soon have to adjust to something new in the airport security line _ giant machines that that scan their bodies for anything a terrorist might use to cause harm during a flight.

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.

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Q: How do these full-body scanners work?

A: The two main types of scanners are "millimeter wave" and "backscatter" machines. Millimeter wave units send radio waves over a person and produce a three-dimensional image by measuring the energy reflected back. Backscatter machines use low-level X-rays to create a two-dimensional image of the body.

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Q: What sorts of things can they find?

A: The machines are designed to uncover what a physical pat-down could turn up but a metal detector wouldn't find. That includes plastic or chemical explosives and nonmetallic weapons in a pocket or strapped to someone's body.

The machines would also find guns, knives and other metallic objects that would set off a metal detector.

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Q: What can they not find?

A: Generally, the machines can't find items stashed in a body cavity. So the scanners wouldn't stop at least one common smuggling method used by drug traffickers.

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Q: How common are they at airports?

A: Because of fears that the scans infringe on travelers' dignity by revealing bodily contours, European officials have generally limited the machines to tests in airports or train stations.

In U.S. airports, the Transportation Security Administration has begun expanding the use of full-body scanning machines, although passengers can opt for a physical pat-down instead. The TSA has 40 in place, just bought 150 and plans to buy 300 more.