A professor was preparing for his lecture course at the start of the semester.
On the first day, he got to the lecture hall five minutes early to drop off his hat. He then left to pick up some things from his office, then went back to the lecture hall to start class.
But when he got back, he was surprised to see that many of the students who were there when he dropped off his coat and hat had left. Afterwards, he sent out an email to the entire class asking why the students left.
One student replied, “We saw you drop off your hat and then leave. So we thought you left for good, and the lecture was cancelled.”
Annoyed, the professor emailed the students, “When a person has left their hat, please assume that the person is still in the room!”
The next day, he walked into the lecture hall to find it empty, but with the students’ hats on each chair.
