Some incidents have been the result of travelers being permitted to carry either weapons or items that could be used as weapons on board aircraft so that they could hijack the plane. Travelers are screened by metal detectors. Explosive detection machines used include X-ray machines and explosives trace-detection portal machines (a.k.a. "puffer machines"). In the United States the TSA is working on new scanning machines that are still effective searching for objects that aren't allowed in the airplanes but that don't depict the passengers in a state of undress that some find embarrassing. Explosive detection machines can also be used for both carry on and checked baggage. These detect volatile compounds given off from explosives using gas chromatography.