The Teacher's Lesson

A teacher gave a lecture  in the classroom
Then he asked: "Did anyone understand what I said?"
The students who didn't understand the lesson
Agreed among them what they should say:
Half of the class  would say "yes" and half would say "no"
When the teacher asked the same question again
As agreed, half the class said yes and half said no.
The teacher was happy, then he said:
"Those who said yes, explain to those who said no.
Everyone was happy including the teacher
While actually no one understood.