In years past, boarding an airplane was a relatively painless, fast experience. Metal detectors were standard, but there was no removing of shoes or having to say goodbye to loved ones beyond the passenger gate. The terrorist attacks of September 11, 2001 changed all of that. Today, effective passenger screening can be, literally, a matter of life and death. Metal detectors, for all of their conventional strengths, "miss" a whole passel of things a terrorist might be able to sneak onto a plane full of passengers. Seemingly harmless-looking liquids, for example can be lethally explosive.

Some screening technologies can automatically detect liquid explosives -- even if they're in sealed bottles. These detection technologies each have some limitations, however. Some use scattered lasers that hit a target and then reflect back to a receiver device for analysis. Lasers can register the molecular properties of the sampled materials and compare them to a database. The problem with laser devices is that their beams can't get through opaque containers, like ceramic or metal. Another method for detecting liquid explosives involves software that teams up with X-ray machines to compare the pixels of whatever is in your bottles to explosives' pixel signatures.

Yet another approach to scanning involves air. In most airports, you're told to walk through a metal detector after sending your carry-on bags through a scanner. Other times, you walk through a detector that blasts air at you. Those blasts of air are used to detect trace particles of explosives. Generally, if you spend time making a bomb, vapors from the explosives stick to your hair, skin and clothes. The "puffer" blasts air at you and then sucks that air back up. It analyzes the air for any explosive particles that may have blown off of you.

X-rays, meanwhile, are once again in evidence in one of the newest scanners. Recently, full-body scanners have been deployed in some airports. The passenger stands in front of the machine while it snaps a full scan of his or her body. The controversial scanners (some think them a bit too revealing of the passenger) use weak X-rays that can reveal images of any hidden weapons beneath a prospective passenger's clothing. The radiation exposure is minimal, and the X-rays go about one inch (2.5 centimeters) under a person's skin. The rays bounce back in the opposite direction, where a camera captures the image. While the scanners are certainly more effective than a standard metal detector, some fear they might not be able to find weapons hidden in body cavities, making them less than 100 percent effective [source: NPR].