If you see a TSA officer "swabbing" or "wiping" your hands (rather than a liquid spray), they are performing Explosive Trace Detection (ETD). The officer uses a small cotton swab to collect microscopic residue from the crevices of your palms or fingers, which is then analyzed in an Ion-Mobility Spectrometer machine. This process checks for trace amounts of chemicals used in explosives, such as nitrates or glycerin. These particles are often invisible to the naked eye and can be transferred to your hands if you have handled dangerous materials. It is a mandatory security measure that can be triggered randomly or if a passenger's behavior or luggage raises suspicion. Note that certain everyday items like hand lotions, fertilizers, or heart medications (like nitroglycerin) can occasionally trigger a "false positive," leading to a more thorough secondary screening or a pat-down to clear the passenger for travel.