Boys / Girls - how does that get determined?

Sven is creating all sorts of theories about how the world works. Recently he has been interested in what causes a baby to be a boy or girl.

First, he tried the theory that a baby becomes a boy if the parents give it a boy name. But when we told him that the opposite was true, i.e. a boy is given a boy name because it is already born a boy.

A day later he said "I know how a baby becomes a boy or girl. If the Mommy eats lots of food when the baby is growing inside, it is a boy because boys are bigger." Not a bad theory.