Mr. M was not joking

When B was in high school, his algebra teacher, Mr. M, was quite serious about bringing your book to class.

"You may think it's ok to leave your book at home.  It is not ok.  The only time you have permission not to bring your book to class is if you hear a knock at your door at 7am, and you open the door, and I'm standing there in a pinstripe suit and hat and carrying a cane, and I say to you, 'Student, leave your book at home.' And then I'll get in my car, and tip my hat, and drive away."

When B laughed at this, Mr. M looked at him and said, "You think I'm joking?"

Three months later, at 7am, B heard a knock at the door.  He opened it and there was Mr. M, wearing a pinstripe suit and hat, and carrying a cane, and he said "B...I will." And then he got in his car, tipped his hat, and drove away.