You constantly have to be alert to several different things. Things like making sure you stay on course (maintain the right heading), the altitude, the airspeed...all of which require movement of the controls. You have to continuously scan the outside for traffic and obstacles, look for landmarks & follow the map to make certain that you're not lost, monitor and communicate on the radio's. All of that + pre-flight planning, inspection, obtaining weather information, and landing & take-off portion of flight.