Was doing my preflight one day which involves inspecting the props for any damage. Was spinning the prop for inspection (the EMB is a free turbine) and noticed a passenger staring at me with a very weird look on his face. Got back into the flight deck, and the passenger demanded to talk to us when he boarded. He stated "I can't believe you're gonna put all of us in danger by flying this thing! I saw you spin that prop freely out there, that engine has no compression!". Hard not to laugh....