When male teachers do their own birth

The teacher asked the class in math puzzle.
- If last year my twin 2 sons, 1 daughter extra this year, I have how many children?
The class was very quiet, well-known fly hear. Seeing no one raised their hands, he said:
- TI, said he heard what?
TI confused:
- Teacher does not have any children.
He was surprised:
- Hey Look, last year 2, year 1 ...
Assertive little shook his head:
- Dear no sir ... My mother says men of childbearing are not forever.