May I Come in Sir?

A teacher was checking the home assignments of students in the class. A student who had gone outside appeared at the gate.
Student: May I come in sir?
Teacher: ...{silent}
Student: May I come in sir?
Teacher: ........{silent, still checking assignments}
Student: Excuse me sir.
Teacher: {still not listening}
Then the student entered in the class and went near the teacher who was still busy in checking assignments.
Student: {nearly touching the ear of the teacher} Excuse me sir.
Teacher: {suddenly looking at the student} Yes.
Student: May I come in sir?
Teacher: Yes, come in.