Mrs. Green is an old lady. She travels often and is not afraid of flying. One day she is going from Chicago to San Francisco in a big plane. There are a lot of empty seats on it.
Mrs. Green's seat is near a window. There is a young man on the other side of the aisle. He is near a window, too. Mrs. Green looks at the young man several times.
"He's always looking at the engine outside his window," she thinks. She gets up and walks around the plane for a few minutes. Then she sits down and looks at the young man again.
"Yes," she thinks, "he's looking at the engine all the time."
After half an hour Mrs. Green goes over to him and says, "Take a walk around the plane, young man. I'm going to watch that engine for you for a few minutes."