Lightman is using the guilty knowledge technique, mentioning something that only the guilty not an innocent person will know about and watching for who shows a reaction. This technique is sometimes used in polygraph exams: ‘was the person strangled, shot, stabbed, bludgeoned to death?’ Only the killer knows and is likely to show a physiological reaction when the actual weapon is mentioned. Often the newspapers reveal so much about a crime that this technique can’t be used because everyone knows everything the police know.