A cop pulls you over for a minor traffic violation. After giving you a warning or a ticket, he says, "Drive safe!" and starts walking away. But he immediately turns around and walks toward your car again, saying, "Hey, can I ask you something?"That maneuver, known as the "Kansas Two-Step," is aimed at evading the Fourth Amendment's constraints on searches and seizures. Police are not supposed to continue detaining you after the ostensible purpose of the stop has been accomplished unless they reasonably suspect you are involved in criminal activity. The two-step is designed to extend the encounter by making...