A man watches his friend walk slowly across a room, go through a door at the other end, and slam the heavy steel door behind him. The man knows that his friend is dead. There are no windows in the door or in the walls around it. His friend wasn't dieing as he walked across the room. The room on the other side of the door was perfectly safe, and there was no one in it. How did the man know that his friend was dead? Also, if the same thing had happened now, the man wouldn't have died.
They were diving, wearing the old-fashioned diving suits that have a tube that goes to the surface. The were exploring a sunken submarine. When the man shut the door, it cut the air tube, and he died.