Probability is the likelihood that an event will happen.
The probability of becoming a boy is 1 out of 2 and the probability of becoming a girl is 1 out of 2.
Inside of our cells, we have these things that are called chromosomes. Chromosomes control whether we become a boy or a girl.
Mom's have two X chromosomes in all of her cells and dad's have one X and one Y chromosomes in each of his cells.
Before a child is born, both parents give one of their chromosomes. The mom will always give you an X but if the dad gives an X too, you become a girl. If the dad gives you a Y, you become a boy.