Still, screening technology is an inevitable, and necessary, component of the push back against terrorism. Here are some of the most sophisticated methods for detecting liquid explosives currently under development, and some of their limitations:

PINPOINT IMAGE ANALYSIS There's more information buried in the pixels of an X-ray image than the human eye can see, says Steven Lancaster. So why rely on a checkpoint agent to decide if that's really suntan lotion in your bag? PinPoint is currently being tested against explosives at the TSL. The software piggybacks on X-ray machines currently in use but would itself cost around $50,000.

Caveat: Not yet tested on liquid explosives or their precursors.

TERAHERTZ IMAGING Long, penetrating terahertz waves (between microwaves and infrared on the electromagnetic spectrum) can see through packaging, plastic, shoes, and clothes, making concealed explosives stand out against an image of the naked body. That's not necessarily a good thing, according to outraged Heathrow passengers involved in a 2004 trial run of similar technology. British developer TeraView's CEO Don Arnone says it's possible to scramble the naked pictures so screeners can't see them, yet still have the computer detect an out-of-place object in your underwear. The technology can also identify some substances, liquids included, by the way they absorb terahertz waves. "Water, moisturizers, gasoline, and petroleum products have signatures," Arnone says, but there is "a whole class of liquids that don't," including acetone. Lancaster of Guardian scoffs that with terahertz "you can detect organics, but you can't tell the difference between an apple in one pocket and a ball of C4 [a plastic explosive] in the other." Each machine costs over $100,000; a shoe scanner is also close to commercialization.

Caveat: Leather wallets confuse it, and modesty may foil it.

NANONOSES So far, the best explosives detector is still a dog's nose. "But some days they're sick or tired," says bomb detection expert John Parmeter of Sandia National Laboratories. "Our technology is the only thing that rivals a dog in terms of sensitivity," chemist Timothy Swager of MIT claims. Aptly named Fido, this detector can even smell a vapor coming from a closed bottle. Here's how: Attached to a gentle vacuum cleaner is a tiny glass tube lined with a semiconducting polymer. In some versions of the technology, a shower of ultraviolet rays will cause the polymer to "lase," giving off a steady beam of light. But if explosives molecules bind to the polymer, the beam weakens. This warning signal gets amplified over 10,000 times, which accounts for the system's extraordinary sensitivity. While the detector is currently optimized for substances in the TNT family, "you can design chemistries that can detect most any explosive," Swager says, including liquids. Fido is currently deployed in Iraq sniffing for bombs, and its developer, Nomadics, estimates that a commercial unit will cost around $21,000.

Caveat: Each new threat requires developing a new polymer.

MICROCANTILEVERS In a way more like a nose than Fido, these detectors can distinguish between many different airborne molecules. Thomas Thundat, a nanoscientist at Oak Ridge National Laboratory, says the detectors contain an extremely sensitive array of "very miniature diving boards." Each diving board, or microcantilever, is just one micron thick and carries a different coating. When airborne molecules bind to the coating (each attracts a different kind of molecule), the surface tension changes, making the cantilever curl up like a piece of paper that's been wet on one side. The overall bending pattern indicates what chemicals are in the air. "We can detect everything—explosives, chemical agents, biological agents," Thundat says, including vapors as dilute as a few parts per trillion. "But selectivity is still a problem." Unrelated items can set off a detector's alarm, making checkpoint use impractical. Ultimately, Thundat hopes for a device similar to a handheld metal detector: something the size of a calculator that would scan passengers in 10 seconds or less, at the bargain price of under $1,000 per unit.

Caveat: The technology has been in the research stage for more than 10 years.

PUFFER MACHINES Already installed in 37 airports around the country, these walk-through portals aim brief gusts of air at passengers' clothing. The goal is to dislodge any particle remnants of the bomb-making process, including residue from liquid bombs, and carry them aloft to an ion-mobility spectrometer, where the amount of time it takes a molecule to move down a 10-centimeter electrically charged tube gives away its identity. Could a terrorist subvert this system with meticulously clean methods? "What do you think?" retorts nanonose developer Swager of MIT. Each trace-detection portal, as they are formally known, costs up to $160,000.

Caveat: Closed bottles and sterile technique could conceal particles.

While developers tend to champion their own technologies and criticize others, everyone agrees that a single type of detector won't do. "The ultimate sensor will have multiple technologies in it," says Swager. Still, "the most important thing we have in terms of finding terrorists are the people," he adds. "Savvy, well-trained, alert people—there's no substitute. We can't let ourselves get sucked into the idea that we can 'technology' our way out of this."