We like to begin an introductory physics course with a law of physics that applies to everything, has no known exceptions, and whose consequences are already familiar to students. That law is the principle of relativity. By focusing on the principle of relativity itself, and a careful selection of the thought experiments, we can comfortably introduce the basic concepts of special relativity that we will use later in the course. This allows us to construct an introductory physics course that includes 20th- and 21st-century physics as we go along, rather than shoving modern physics off the back end.