Biology class

During a Biology class, the teacher asked the class,
Why is it that during childhood girls tend to grow taller than guys?
Little Johnny raised his hand and replied,
That’s because guys have balls and that weighs them down.
The teacher, a bit annoyed, responded,
Then why is it that at maturity guys tend to grow taller than girls?
Little Johnny countered by saying,
That’s because girls get breasts and they are heavier than the guy’s balls.