It used to be that in order for a college student to receive credit for a particular course, a card that listed his or her courses had to be signed by the instructor/lecturer. It was, at the time, policy that students attend their courses. But depending on the size of the class, it was often possible to receive credit, even after not attending the class regularly.
Not so with this physics professor. If he didn't recognize you, you would have to repeat the course. On one occasion, a student handed his card to the professor to be signed. The professor looked at the name, then at the student, and said, "I've never seen you in my class," and handed back the card.
Now being a quick thinking science major, the student proceeded to the end of the line. When he was at the front again, he handed his card to the professor.
The professor looked at the name, then at the student, and said, "OK, you look familiar," and signed the card.