Examination

    An examination. A student responded perfectly to all questions. However, the professor decided to nag him and asked, “The last question. How many lamps are there in the room?”
“20,” replied the student.
“You’re wrong. They’re 21. I carry a flash-light in my pocket.”
The student had to resit. The professor asked again, “How many lamps are there in the room?”
“21!” replied the student.
“No. They’re 20. I did not bring my flashlight.”
“Yes, but this time I brought mine,” replied the student.