Custom at Duquesne University dictated that if a professor was ten minutes late, class was canceled.
One professor arrived early for a 9 a.m. lecture. He placed his hat on his desk, and went to the faculty room.
Before he knew it, it was 9:10. By the time he got back to his classroom, it was empty.
The next day, he let his students have it. "When my hat is here," he fumed, "I'm here!"
The following day, the professor arrived at 9 a.m. He was met by the sight of 28 hats on 28 desks -- and no students.