"Half awake" joke

n this particular branch of the Army's officer training school,
the instructor was returning a test. The students identified their work by the last four digits of their Social Security number. In the early hours of a morning, the instructor was calling the numbers.
.
"Four-seven-seven-zero?" he asked.
.
"Here," replied one half-awake lieutenant-to-be. Taking the
paper, though, he realized he had mistakenly asked for the
wrong paper.
.
"Seven-zero-seven-five?" asked the instructor.
.
"Here," repeated the student, gearing for trouble.
.
"I thought you were four-seven-seven-zero, soldier," spoke the teacher.
.
"That's right, sir," answered our hero. "I have a nick-number."