Many of the U.S. Air Force's formal and informal traditions are an amalgamation of those taken from the Royal Air Force (e.g., dining-ins/mess nights) or the experiences of its predecessor organizations such as the <answer>U.S. Army</answer> Air Service, <answer>U.S. Army</answer> Air Corps and the <answer>U.S. Army</answer> Air Forces. Some of these traditions range from "Friday Name Tags" in flying units to an annual "Mustache Month." The use of "challenge coins" is a recent innovation that was adopted from the <answer>U.S. Army</answer> while another cultural tradition unique to the Air Force is the "roof stomp", practiced by Air Force members to welcome a new commander or to commemorate another event, such as a retirement.
What organization did the US Air Force adopt "Challenge Coins" from?
U.S. Army